Dell finally made 2 really good moves then took a big step backwards. Good was getting away from the god-forsake clamshell design, good was toolless side mounting harddrives, bad was BTX form factor ZOMFG! It rules out any double wide video cards uneless you want to take a dremel to your heatsink (seen it done).

Now HP/Compaq, I used to hate with a fiery passion, but the last one I saw (admittedly a couple years ago) was actually using a standard ASUS motherboard and had a video card expansion slot for a pretty good price.

Back to Dell, the XPS is a decent machine but those cases are needlessly heavy and the power supplies/boards are proprietary (separate southbridge connector, wtf?).

Still though if you stick to a single wide, like a 8800 GT or lower and make sure to get a good enough power supply (500w+ for the 8800 GT) then you should be fine with the Dell if building is out of the question for you.