The most important thing is lots of memory - 1 GB should be OK, but more would be better.
Then a large hard disk; 2 SATA diska would be better than one; another small hard disk to act as the swap ile disk (virtual memory) would also be useful, otherwise make a small (2 GB) partition on the drive that DOESN'T have the sysem on it and use that for the virtual memory. You could set up the 2 hard disks in a striped RAID which might be faster, but I am never confident about striped RAID - if one disk fails, you lose the lot!
Finally, a fast processor. I have heard that Pentiums (Pentia?) are faster for this task. Again, if you are serious about this, consider a dual-processor set-up, but make sure that the software can take advantage of this first.
I can't advise on the graphics card - you might be better off with a more specialised card; you don't need the latest and fastest nVidia or ATI chipsets.