If you have money then go on, go for GTX480's and SLI them...
I doubt it will bottleneck by your OC'ed Phenom II x6...
You just running 1 monitor? so i think single GTX480 will be enough.
But, if i was you, i would go for HD5970's and CF them...
I've decided to go with this setup
-Antec P182 case (my current case, ill just be swapping out the components)
-Phenom II X6 1090T OC'd to 4.2GHz
-Coolermaster V8 CPU cooler
-8GB DDR3 1333MHz
-Kingwin 1,000 Watt PSU
-2 TB of HD space
-Asus M4A79XTD EVO AMD 790X
-(2) 5970's in crossfire
I'll probly buy one 5970 for now, then in about 3 months buy the second one, plus another monitor.