It should be okay. My dad runs a 3.2GHz Pentium 4 Dell with two hard drives, a gig of ram, and a 6600 with the stock 250W supply. Which is why we've been saying several times that the amp ratings are actually more important than the raw wattage The ATI and nVidia recommendations probably err high for additional ass-coverage on their part.
Keeep running for a while (perhaps run a stress test on it) and then blame any odd behaviour on the PSU. Things like random reboots, and mine once when it had a bad PSU would randomly turn on in the middle of the night....that was interesting...
You're going to need a better psu for anything decent. A new MB for anything newer. I'd say 6800GToc at lowest to be honest.
It's cut out of two games on me so far, but they were unstable to begin with (unpatched Far Cry and Deus Ex: IW). Everything else is running smooth as silk. I'm leaving it in the Dell for the time being, and I'll start building my own computer soon.
Before you start on that, I suggest you wait until Unreal Tournament 2007 comes out and build a PC that will handle that game either very well or "mid-range" for that game, with all the eye-candy turned on. It is much better than buying now and finding out when that game comes out, all the hardware you bought that was "top of the line" is now considered low-end.
