I used to have an XPS 420 that I modded a bit.
IIRC it had a Q6600 2.4Ghz quad core, 8GB ram, and a 896mb GeForce GTX260 Core216. It was pretty fast back in the day. I remember the only way you could overclock the processor because of the locked bios was by putting a strip of electrical tape over one of the processor pins. It would bump the clock up to 3ghz, but since you couldn't adjust voltages and the stock cooler wasn't that great it would get very unstable.