2x Radeon 6450 or GTX580

I'm guessing you mean 2x Radeon 6950's, which will outperform a single GTX 580. All the same, given the number of complaints I have read about the stability of Crossfire drivers, shimmering issues, and microstuttering; I would go for the best single GPU, the GTX 580.

The 6450 is an OEM card and isn't even capable of crossfire so I'm guessing that's a typo? If 6450 crossfire actually exists it's still not going to be 1/3rd as fast as a 580.

I would get 2 5870s.