redgarl :
Do yourself a treat and get a CF config of two 6850 and OC them.
If you want to go single card, go for a 6950.
@Cnox, I don't know about you, but my twin 6850 are the best investment I made in term of graphics. I am happy I got them for The Witcher 2, but I admit that amd should really kick their butt for releasing a profile on the launch date of the game.
As for performance, it is quite impressive.
I'm not arguing that when it works, it's great. My argument is that you can rely on a 100% working platform with a single high performance card than a less than 100% working platform with SLI/Crossfire. My exposure to dual GPU configs has been confined to AMD tech, but I believe SLI has the same limitations WRT application profiles. It's interesting that AMD would describe a 'fix' to negative scaling in games was to disable crossfire in the game.
I don't know about you, but I play a lot of different games. I think about half of them work under Crossfire. I think that's pretty bad. Add to that the fact that crossfire disables automatically when running in a window (I don't believe this is an issue for single GPU configs) for me that's just another pound of BS to add to the pile.
DX10 and DX11 supposedly add support for dual GPU in window mode, but the DX10 titles I've tried did not have it work (apparently 'supports' and 'does' are 2 different things in a game developer's eyes).
Here's another pound: a single card can run in 16x PCIe speeds, but go dual gpu and it's 8x 8x. Unless you pay a premium for your motherboard...
So, I'll just restate it: Skip the SLI/Crossfire, get a beefy card instead, put the money saved in extra power supply, additional card purchase and expensive motherboard to offset the cost of getting the higher quality single card. At the end of life of the machine, you'll just have 1 POS card to get rid of instead of 2.