Yep, go with the single card. GTX 295, or a 4870x2. Go with the better deal, or the one that comes with a better game/deal. I'm no fanboy, so saying which one is better is negligible. Physics/Cuda on the Nvidia graphic cards are doing way better than the equivalent of ATI graphic cards, so that could way in your decision.
If you have time to wait, you could wait for the DX11 cards coming from ATI later this year/early next year. Expected to be way faster than the current DX10 cards while being at the same price point as the 4850/4870/4870x2 when they came out.