I want to ask one moar question
2x GTX 550Ti or 7950 ?
A 7950 would completly rape the 550ti, the 500ti was always a under performer, and sling two mid-low end cards is never a good idea, will give you tons of headaches, 2x 550 tis give around the performance of a gtx 570, and the 7950 even rapes the gtx 580, (as the 580 is a last generation card) not that the gtx 580 isn't a decent performer still, and is still a viable buying option. but two rules are always to buy in generation, and never sli/cf two low end cards thinking you will get a high end experience, the 7950..or a gtx 580, or even if you can afford to throw like 30-40-50 more dollars at it, the gtx 670 is a better option.
Another note, if you can wait out a bit longer to make your buying decision, the radeon 8xxx series is about to be announced....so enuf.