Here is an interesting article that points out the pitfalls of multiple video cards, even the dual GPU cards that are made. The article is about the titan, but this post is not about that card:

http://seekingalpha.com/article/1243621-why-nvidia-s-titan-keeps-selling-out?source=yahoo

It appears that even though there is twice the pixel processing in a dual card setup, overall performance suffers since the GPU's still can't be synched to work properly together. This seems to support those hardware posts I see where people wonder why a SLI or Crossfire or Dual GPU card does not seem to perform the way that it should.

So, does your dual GPU card set up leave you wanting for more?

Did going to a dual GPU card gain you any appreciable performance in a game? Not benchmarks, but a game?

Bsically what I've been saying all along. SLI and Crossfire only make sense as a stopgap solution to extend the life of a build once performance has tapered off due to it no longer being the latest and greatest hardware. By that time the cost of another GPU of the same model is much less, so it can be a viable life-extension strategy rather than buying a newer GPU. This is compounded by the fact that some games just don't play nice with SLI/Crossfire.

But when building a rig, it's always better to just get a more powerful single GPU than go with Crossfire/SLI