Whenever I buy Dell, I buy a barebones system, then add parts myself. Why? Contrary to what LikeLinus said, it is often far cheaper that way.
For example, you might want a specific computer with 1 GB of memory. Dell sells that computer but charges say $300 extra to go from 128 MB to 1 GB. Well, I'll get a computer with the base 128 MB from Dell, then add in the 1 GB of memory myself for $100 and be on my way saving $200 in the process. Plus I take out that 128 MB that I could sell for extra money or use for spare parts on other machines.
Same goes for video cards and Xeon processors. It is far far cheaper usually to get those yourself and put them into the bare Dell box.
Everything is fairly easy in a Dell box to upgrade. There are a few exceptions:
[*]Motherboard. Dell has non-standard cases and motherboards are hard to upgrade. But, if you really wanted a specific motherboard, why are you getting a Dell in the first place?
[*]Power supply for P3 computers. Dell used a nonstandard power supply in the P3 days.
[*]Large sized power supply for P4 computers. Dell uses a strange case, so putting in an aftermarket power supply often isn't easy. But Dell uses high quality power supplies and I've never needed to upgrade one.
[*]Floppy drive. If you get a Dell without a floppy, you often won't get the mounting hardware to put in your floppy. Last time, I just got a strip of scrap steel, drilled 2 holes, and in 3 minutes I had mounting hardware.
[*]Processor fans. Dell's don't overclock, so why you need a different fan is beyond me. But if you do, it isn't easy.
Everything else is a piece of cake and can save you a bundle in many situations.