Shwell...
The desktop PC that I use for SH has the following:
Intel Prescott Pentium 4 530 (socket 775 LGA) which has a core speed of 3006.8 Mhz and a 1024 KB L2 cache. Bus speed is 200 Mhz and FSB is 800 (ish) Mhz.
Mainboard is an ASUS P5GD1 with 2 gigs of RAM on it.
Graphics card is a PCIx Radeon X800GT
It has three hard drives, one of which is a Lacie (F.A.Porsche) USB external, the other two internal ones being a Western Digital and a Seagate. There's a lot of other stuff plugged in, such as Track-IR, printers cameras etc, so it has a heavy-duty PSU.
It also has a lot of (slightly noisy) additional cooling (not a big deal with headphones on).
Runs recent games and sims such as FSX, SH4, ARMA etc with no problems whatsoever (ever so slight slow down when everything is cranked up in SH4, but nothing you couldn't live with), then again, I did build the thing myself and chose the components very carefully so they would work well together and this might have something to do with that, as it is hardly cutting edge these days. I have a lot of other PCs and some Macs, some of which are newer and better specced than the above, but since this one seems to handle stuff okay, it's the one I use.
Chock.