ATC9001 lists a really good rig.
My only change would be to get a really good Network Card. If you're talking about a T1 and 64 people connecting to it, don't skimp on the network card. Get yourself an Intel Pro1000MT. It is a 66MHz card, costs $50, and will give you lots of bandwidth at very low latency (since it uses the 66MHz PCI slots.) It is a 10/100/1000 card, autosensing, of course.
If you don't have to live next door to this computer, and don't care about noise, then get any old good, noisy heatsinks. The Speeze 5R265B1H3 that Joe reviewed a little while back is actually a really good heatsink for $8. I bought a pair, just to test them out. The noise level is much better than most heatsinks I have used they perform really well. I ran a pair of XP1600's overclocked to 1667 Mhz, and my temps never went over 110 at full load. For my own use, though, I prefer SVC GC68's with Panaflo L1A fans on them ($14 combo.) This combo is nearly silent. Not as good at cooling as the previously mentioned Speeze, but very, very quiet. I run my wife's dual XP1600's overclocked to 1533 MHz @ 1.525 Volts with these. Temps are grerat and I run three instances of
[email protected] 24/7. Can't overclock the machine as high as I could with noisier heatsinks, but for me, the performance is great, at a noise level we both can stand.