The Tech Report has tested multi-GPU performance with ATI and NVIDIA's latest DX11 cards, you can check it out over here.
Multi-card graphics solutions can make more sense inside of a desktop gaming rig than you might first think. For instance, a pair of graphics cards can use twice the area of a single, larger card for heat dissipation, making them potentially quieter, other things being equal. Two mid-range graphics cards will draw power from two different PCIe slots, which may save you the trouble of having to accommodate a card with one of those annoying eight-pin auxiliary power connectors. And these days, the second graphics card in a pair is generally pretty good about shutting down and not requiring much power or making much noise when it's not in use. Add up all of the considerations, and going with dual graphics cards might be less trouble than some of the pricey single-card alternatives.
The value equation can tilt in the direction of multiple cards, too, in certain cases. Let's have a look at some of the options we're faced with in the current market, and then we'll consider more specifically what combinations of cards might be the best candidates for a pairing.