Re: GTX480 SLI + 750i + q9550 3.8GHz
I had a dilemma very similar to yours couple of months ago.
I ended up replacing my q6600 @ 3.6 on 680i sli mobo with p55/i5 750.
Surely you'll get a lot by adding another gtx 480, but you'll also get a lot by replacing your mobo and cpu, especially in minimum framerate department and that's where it really counts.
btw. i run gtx275's in SLI.
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