I think Lonepig did a mistake about the PCI-X thing.

PCI-X is like PCI: slots for periferal cards, not video card. Unless I am mistaken, there is no possibility for sli card there.

The new standard for video card will be PCI Express which has nothing to do with PCI-X. I dont know which one came first but its causing such a confusion that the idiot who picked the naming of this standard should be fired...

You didnt say what was the intended use of the computer so I will guess its 3D and will shut my mouth about getting a mac .

If you want big performance gain, think about the Raptors HD. As it has been pointed out, putting these in RAID will improve performance of diskbound operation by a huge factor and general operation by more than what any PCU upgrade could do. You dont have to get the 2 drives right now, just getting one now and be on the look out for another when you feel the need.

Finaly, look for the highest FSB you can, this is the bottleneck of most operation that arent diskbound.