To do SLI you _must_ get a Mobo with an nvidia chipset. A quick run-down:
Assuming you're going intel:
680i - C2d, Quad, SLI x16 (ie both cards use full 16-bits of the PCIX bus)
650i - C2d, Quad, SLI x8
All other nvidia chipsets dont support c2d. But 590 and 570 will still do SLI (x16 and x8 respectively), but you can only get core2, pentiumD etc.
Full details here
Assuming you're going intel:
680i - C2d, Quad, SLI x16 (ie both cards use full 16-bits of the PCIX bus)
650i - C2d, Quad, SLI x8
All other nvidia chipsets dont support c2d. But 590 and 570 will still do SLI (x16 and x8 respectively), but you can only get core2, pentiumD etc.
Full details here