Re: GTX480 SLI + 750i + q9550 3.8GHz
Your current platform will be bottleneck for two gtx 480's, even with 8xAA.
PCI-E slots at x8 should not be a concern, but the platform itself is the limiting factor for such a high end graphics cards.
When you're running gtx480 in SLI you need as much CPU power as you can get.
Also, Zotac AMP gtx480 cards are three slot cards, and you won't be able to fit 2 of those unless you buy EVGA Classified or ASUS Rampage III Extreme( if you want your cards to be in x16 slots) and computer case with 9 expansion slots such as Coolermaster HAF-X.
EVGA 680i A1,BIOS P30
Core2Quad Q6600 G0 @ 3.6 Ghz (400 x 9), Ultra 120 eXtreme;Vcore 1.4125V
4 x 2GB Super Talent 800 Mhz
Leadtek GTX 275 SLI
WD Raptor 150 GB, WD 1TB SataII x 2
Lite On 20x SATA DVD-RW,Pioneer 111D DVD-RW
TT Toughpower 750W
Antec P180B,6 x 120 mm case fans