I have personally been inside all these systems and can say authoritatively: they are NOT the same. They are very similar on cursory visual examination. The chassis are very similar, and the motherboards are the same shape. But they are not the same.
Same: They are the same size. The parts bolt into the same chassis locations and the I/O ports fit, mostly. The same processors (up to T7600) can be used in all of them, after upgrading to the latest BIOS revision. The same RAM can be used. The optical drives and daughter cards (except video cards) are all cross-compatible. None of these can run their memory at 800MHz but all can use 800MHz memory sticks, which will automatically run at 667MHz. They can all run Windows 7 without major problem (there are minor problems, the worst being Fn/brightness control), but video card drivers are going to be problematic. Since they can’t utilize 4GB RAM, it is simpler to use 32-bit Windows. 32-bit drivers are easier to get working right than with 64-bit Windows which is pickier about its drivers, and driver support from the various component manufacturers is spotty at best for 64 bit. Linux and early Mac OS X may “work” but you will have issues with drivers and glitches, even after you hack the installation onto the machine (and hack is the appropriate word in the case of Mac OS). The top and bottom panels are all metal, on all these model variants. #yesallmodels. The build quality is the SAME, none are made with specially higher or lower quality materials or hardware. The fans are interchangeable (CPU fan is larger, GPU fan is smaller). The 9400 and e1705 in some cases are identical except for the trim panel over the keyboard with a different painted-on model number.
Graphics: Some e1705/9400 notebooks have a motherboard that supports either add-on (ATI/Nvidia) GPU cards, or else they have the Intel graphics. If the laptop had the integrated Intel 8MB graphics from the factory, you can NOT upgrade to any of the ATI or nVidia cards, ever, at all. The motherboards are dramatically different. The connector for the add-on video cards is not installed on the Intel graphics mobo, and the header to the LCD screen is on the motherboard. There is no GPU fan, GPU fan connector or GPU fan drive circuitry on the lowest-powered-graphics version of this computer. The connector to the LCD is on the add-on video module in computers with separate GPUs, and there is NO connector on their motherboards for the video out to the LCD. You can NEVER downgrade from add-in to integrated video without changing the motherboard.
Video card support: some versions (Precision) have native BIOS support for the highest-end Quadro FX 3500m video card. The others will not play nice 100% with these cards. They may be mounted, but at least brightness will give you difficulty. The GPU heatsinks vary also, but are usually interchangeable. Video cards from the Inspiron 9300-based PCs are radically different – they do NOT fit physically, at all. Even their heatsinks are different to the 9400-based GPUs.
Chassis: The Intel graphics motherboards may have a chassis that will NOT work with an add-on GPU. The fan grilles are cast into the right side of the base so they look the same from the outside, but they were not machined (or cast or milled or otherwise finished) so that the ventilation slots are open. These slots are BLANK inside the computer, not holes. Also, the bases that have capacity for add-in GPUs with dual heat pipes have an extra notch cut into the area where the heat sink for the CPU goes to the back of the case. This is an air baffle that will confound attempts to install a dual-pipe GPU without metal modifications. The Precision and XPS, and maybe some Inspirons, can also attach to a docking station. The base model Inspiron has no cut-out on the bottom for the connector, and the connector is absent from the motherboard.
Displays: they are all cross-compatible, except that the XPS display module will only shine the lights on the top panel when used with an XPS motherboard. The displays of different resolution and different “glossy/matte” variations all have the same footprint and cabling, and will plug right in and work. The LCD panels can be changed for one another. They all bolt in the same way. You don’t have to do anything special to get the computer to recognize a different display module. The Precision has a black trim panel around the display, vs. metallic light gray on the others. The bottom of the rear panels have different wording cast into them to reflect the different models.
LEDs: the XPS base has some (frankly, really neat) LEDs that can change to 16 different colors. This is a feature only found on the XPS motherboard. The connectors and controlling hardware are not there on the other boards. Also the XPS speakers are different (with LEDs) and the LED modules themselves mount to the fans. The non-XPS models don’t have any of the extra color/bling LEDs at all.
BIOSes: BIOS software is different from one model to the next, and you cannot interchange these unless you are t3h l337. This is not an open question as far as I can tell. Some of the earlier versions had limited RAM capacity (2GB) but latest revisions have the ability to “see” all of 4GB of RAM on all models. As these have the Intel 945-family chipset, so NONE are able to use all of 4 GB. This is a hardware limitation of the chipset that is not upgradeable, and cannot be changed. You will never be able to use 4GB+ in these computers. The XPS BIOS has exclusive ability to change some stuff the others don’t, including overclocking on “G” processors with unlocked multipliers. CPU clock speed throttling will occur at higher than stock speeds during heavy use, including frequent and heavy throttling down to base speed or lower, on the highest overclock setting.
Batteries: The higher-power computers came with higher-capacity batteries. They may not boot at all, or may complain about the lower power battery being installed. The lower powered computers will work with the more powerful batteries with increased run-time (win!)
Power supplies: the higher-power computers need 130W or greater power supplies and will complain if they boot at all with 90W or lower power supplies. Generally they will operate with lower power supplies after booting into Windows but will sometimes complain and they will NOT charge the battery. If you have enough battery to boot, you can usually keep running at reduced processor and video card power with a too-small power supply.
Palmrest/touchpad/buttons: the palmrests are interchangeable but some have 2 and some have 3 buttons. The XPS touchpad has a red LED logo in it. The Precision is again different (darker) colors. The removable panel near the hinges is also black on the Precision vs. light gray metallic paint on the others. This trim panel is also interchangeable.
Colors: duh? Obviously there are color differences. The main panel on top of the XPS is very nicely color anodized red or black with machining marks in a pattern. The main area of the Precision M90 top panel is brushed metal medium-gray anodized. The Inspiron is painted. All have painted edges. Other color differences are generally paint.
Fans: the Intel graphics-equipped models have one fan, for the CPU cooler. Add-on GPU models have two fans, and associated controllers, connectors, and ventilation slots. The fans are interchangeable except that the XPS fans have LEDs mounted on them. The lower-end models run quieter because there is no fan on the GPUs. Higher-powered video cards run hotter and louder with lower battery life.
I have rebuilt and scrapped each of the e1705, m1710, m90 and 9400 on several occasions, and can tell you they are NOT the same computer (except the 9400 and e1705 are SOMETIMES the same). I personally have on my lap just now an Inspiron 9400 with Intel graphics, T7600 CPU, 4GB Ram, Windows 7, and a solid state hard drive. It is plenty fast enough for anything but modern games or video rendering. The SSHD made a drastic improvement as did maxing-out the memory.
Personally I prefer the Intel graphics version. The ATI graphics card is (in my experience) rare as hen’s teeth but works fine. I’d use that if I could find a working one. I intentionally avoid the NVIDIA graphics cards because they will eventually ALL FAIL the same way: they chips internally fall apart from solder failures. I have repaired a couple of these and sold them without warranty, working fine for an unknowable amount of time. I just don’t trust ’em.
TL;DR I know, sorry. I have looked in lots of places and never found this much information on these computers all in one place. IF you have something you want to add, please leave a comment.