With a lot of games I'm gonna tell you to begin with. Many games have issues detecting dual/triple GPU's (i.e. dual cards, 5970, etc.) The 3x5770 would be the best due to raw power (most fps). But the 5800 and 5900 series cards have the power of 2 5770s and are twice as fast. On top of that, a lot of cards that aren't initially designed to run with a certain DirectX will have trouble in the future with them (i.e. A card meant to run DirectX10 trying to run DirectX11 Software).
Since your trying to go for the best thing for the buck from what it seems, I would suggest a 5850 or just blow the few extra hundred dollars to get a 5870 which I myself will be getting
Have you thought of any manfacturer's who you want to buy from?