For most of the time that we have had the ability to use multiple video cards in one gaming machine we have had to use two of the same exact GPUs and card models for it to work. This was the case with both NVIDIA and ATI cards.
LucidLogix has announced that with the unveiling of a new video card architecture called Hydralogix 200 real time distributed processor. The new tech allows any AMD/ATI or NVIDIA GPU to be used at the same time as any other GPU.
That means you can have an AMD and NVIDIA card in the same machine and still get increased performance. The new Unity architecture will include cards set to be on the market in time for Christmas 2010 at under $199 that will include the processor allowing multiple cards on any mainboard.