What I always heard was to avoid Dell because the important stuff to upgrade is proprietary so you pay through the nose to add memory or upgrade HW. |
I have the Inspiron 9300, which was built in March 2005.
I have replaced the stock Pentium M 1.6 Ghz processor with a Pentium M 2.0 Ghz processor with no issues.
I have also replaced the hard drive (originally a 40 GB Hitachi 5400 RPM) with a Seagate 80 GB 7200 RPM, and replaced (2) 256 MB sticks of RAM with (2) 1 GB sticks of RAM (Kingston). I also replaced the stock Toshiba DVD-ROM drive with a Sony DVD/RW drive.
All replacement parts were purchased through NewEgg.com except the RAM which I got through Amazon.com. The only issue was with the Sony optical drive which is not quite flush with the side of the case - otherwise all parts work flawlessly.
Therefore to answer your question; the Inspiron series - at least - has a proprietary motherboard, case, keyboard, touchpad, and monitor.
The hard drive, optical drive, memory, and processor can be upgraded within the motherboard restrictions/specifications. All of these can be purchased from an online vendor if you wish (the cost is much lower).
As for the graphics card, Dell offered upgrades (at that time) to either a Radeon X300 (128 MB) mobility PCI express card, or a nVidia GForce 6800 (256 MB) AGP card. I opted for the PCI express card due to my knowledge of the lower heat build up of this architecture. Due to the lack of availability of the newer mobile video cards through online vendors, it is better to purchase the unit with the video option you plan to use for a few years.
If you purchase a Dell, make sure to note the higher end options specific to the unit you are purchasing for future reference. This will allow you to upgrade with confidence, as Dell is not eager to offer the deeper technical knowledge of their equipment - at least in my experience.

It should also be noted that Dell offers a "Build Your Own" feature in their XPS 700 series where you can purchase the basic parts for a machine and actually build your own by purchasing a basic foundation (case, power supply, Dell BTX MoBo, Processor, and heat-sink/fan). The heat-sink fan that comes with the newer Core2 Duo processors in a heat-pipe style with multiple fins and two aluminum fans - really nice, IMO. The cost is about 30% more than you could purchase and build one with similar components for, but Dell will apparently assemble the basic foundation listed above, which ships with "Free-DOS" only.
The new owner can then install memory, video card(s), hard drive(s), optical drive(s), and purchase their own custom monitor and input devices. A nice option for someone who feels confident with the easier hardware assembly and wants to build a custom rig.
Note - you can also select all needed hardware parts so that you can build the complete PC tower with no operating system installed if you wish.
For all the bad press and fall in profit margins, they are providing some nice options on purchases.