What resolution will you be gaming on?
If you are using a 1920 x 1080 panel, a single nVidia GTX 670 or Radeon 7950 is probably all you need:http://www.anandtech.com/bench/Product/598?vs=550
On the same 1920 x 1080, if you want maximum FPS now and don't mind spending considerably more money, a single GTX 680 or Radeon 7970 is a good option:http://www.anandtech.com/bench/Product/555?vs=508
If you are gaming or want to game over multiple monitors (for example 3 monitors at 5760 x 1080 resolution), I think in general, the more you have, the better performance. That said, I've read that a GTX 680 SLI only outperforms a GTX 670 SLI by a few FPS. And a single GTX 690 is also only marginally faster as well (I don't know anything about a GTX 690 SLI because that is $2k). If money was irrelevant, I'd prefer a single GPU solution though.
I'm not read up on Radeon cards in Crossfire, so I'm hesistant to comment.