We've gotten lots of heat from the community for the lack of multi-GPU support in Windows Vista.  No longer -- check out Multi GPU running in Vista from the Gaming section of Hexus:

"...the reason ATI are pumping out 3DMark over three monitors is to give a visual layman’s term view of how the two GFX cards, in this case two Radeon X1900 cards, work together to produce the image.  The first screen is the normal output, but ATI have forced supertiling so that a non-techie bloke, (me), can see how one card renders one part of the image while the other does another part..."

... and find more coverage in The Inquirer at their story ATI demonstrates Crossfire on Vista.