msi r6850 pe/oc or sapphire hd 7770 oc

[zelda5432]

I want to know, which one wil be better, msi r6850 pe/oc or sapphire hd 7770 oc
i will get one of these, and i haven't many options where i live so please don't suggest a different card

 

[zelda5432]

btw, if this are my pc specs, could it run on a Thermaltake TR2 500w

MSI 880GMS E-35
Phenom II x4 955BE @ 3.9ghz, on CM Hyper 412 Slim
Transcend 2x4 = 8gb ram
Msi 6850 Cyclone PE/OC
WD Green 2tb, Hitachi 1tb
DVD RW
4 casing fans