I would not be using any AA. Just post AA.
Are you going to wait for the new Nvidia cards before deciding what to do? I'm going through the same dilemma at the moment.
i2500k
5870 1GB
Dell U2711
Although 2x7970 would be nice, it's an absolute fortune to crossfire these cards. Hell, even $600 is a lot to spend for mediocre performance.