6950 is equivalent to a 7850. Still a good card (I have two, unlocked to 6970). The Sapphire dual fan cooler particularly good.
And yes, that CM500W will be insufficient, imo, with a 6950.
Go 1080p. Anything lower will be a waste.
Therefore if it started to sound costly, downgrade to a 6850 or a decent second hand GPU. And upgrade later (with a 7xxx or a 6xx card, whatever NVidia brings in your price range).