I was reading some specs on the 2900PRO somewhere, and it said something about needing a "dual-width" PCI express slot, so that was the source of my concern.
Thanks for the input Nos, I think that I might just give it a go if I can get rid of my 8600GT.
That just means make sure you have enough room for the cooling unit, it takes up more room than a single slot card. It's basically double the thickness.