i would recommend a single 5870 , and overclock it a bit .
it would be enough for everything with AA excluding crysis for which you can :-
1.for action gameplay , like completing mission and finishing the game , use slightly reduced settings (gamer , not enthusiast) . (action = high fps) , or u can use dx9 mode and all max settings .
2.for exploring , interacting and experimenting in general , just roaming about , u can use max settings in dx10 .
of course u knew about the settings , this is just a strategy , as u wont notice subtle graphic differences in fast action , and gamer quality is good enough for that .
with cards like 5800 coming out , no need for CF in 1080p . cf cards are harder to sell as u have 2 of em , profile and scaling issues etc .
and cryengine 3/crysis 2 is coming out next year hopefully .. it would run much smoother , use dx11 even on single card setups .