You have a choice of these chipset / processor / DRAM combos:

NV 780a / Phenom / DDR2
NV 780i / Intel Core 2 Quad / DDR2
NV 790i / Intel Core 2 Quad / DDR3

Intel is probably the fastest. To get any real advantage from SLI you need a CPU running at 3Ghz+ tbh, that's either Intel or AMD. DDR3 is more expensive than DDR2, and not much faster by any stretch.

As for which you go for - that's entirely your choice. In this forum, most people would recommend AMD. In the Intel forum, everyone would recommend Intel. In a neutral forum, the majority would still favour Intel, especially since it overclocks so easily for nice boosts with SLI.

Do note that the 780a chipset is the only one that will let you power down the graphics cards when not in use. If you go for a 9800GTX, that's a 54W saving, with SLI that increases to 108W.