Not all the time.
Both Crossfire (ATI) and SLI (nvidia) involving sharing data via the pci bus, which is slow as it has some overhead to split the data for processing to the 2 seperate cards and then reform it back to the outputs.
Ideally you want a card with 2 gpus on the same card, for that the 4870x2 is the best card. At the time of its release its worth mentioning it also beat the top nvidia card in performance.
But for same price of a top INTEL rig, I can build an ATI and AMD rig that will out perform it every time.
Also its worth mentioning that when it comes to cards like the 4870x2 that can get 400 frames per second in some games, YOUR EYES CAN ONLY SEE A MAXIMUM OF 31 FRAMES PER SECOND.
So when it comes to gaming dont worry about having the biggest graphics card combos.
I have a friend who has 4 of the meanest cards NVIDIA makes, and when we play GRID side by side at highest res, you cant tell the difference and I only have a little HD 4650
Xtreame