Nope - not yet at least as far as I know. If you buy crossfire, you need ATI to make it work. I saw some SLI drivers a while back that will allow some support of Crossfire on an NForce board, but nothing the other way around as far as I know yet.
Still the only real way to do it is use SLI on an SLI board and use Crossfire on an ATI based board. Not sure why you are mixing and matching.
-S
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