In your case the PSU is a problem for 2 cards (in all honestly I'd even want a 750W or better) for the 7970 everyone is recommending you get instead of xfire the 6950's!
As for cards it comes down to what you want and what you can get for your current if you do want to upgrade to one of the better single card solutions.
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Comp1 (Mine) : ASUS Sabertooth, i7 2700K, 8GB 2133 GSkill, XFX 7970 DD, OCZ Vertex3 240GB, OCZ Vertex3 480GB, 600GB VelociRaptor, Corsair 1000HX, Cool Master Cosmos 2
Comp2 (Wife's) : Intel DP67BG, i7 2600K, 8GB Corsair Vengeance 1866, 1TB HD, 2TB HD, Sapphire 7950 OC, Corsair 950W, HAF 922
Comp3 (Kids) : ASUS P8P67 Pro, i7 2700K, 8GB 1866 GSkill, Sapphire 7970, Crucial M4 120GB, OCZ Vertex3 240GB,1TB WD Black, Antec 850W, Antec 1200
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