OP, could you run your setup through a benchmark like 3dmark11 or something? Would be helpful to compare your setup to today's cards.
I doubt 5850 CF is as strong as a 7970, since that would mean that 5850 CF would have to be significantly stronger than the 5970 (I don't remember it being that strong but could be wrong).
Sorry I don't have 3dmark11 >.< but the main reason why i'm wanting to upgrade is because not many games really support crossfire fully.
Cod4 will only make my GPU's run at about 25-40% each so i'm not getting what I pay for.
I also don't want to be losing that much performance either, http://www.joystiq.com/2012/08/16/borderlands-2-bundled...
The video makes it seem like Physx is really going to be used in the future.
PhysX is polluted by a bunch of marketing BS from Nvidia.
If you are looking to dump a ton of time into a game that uses PhysX well, then you should absolutely have an Nvidia card. However, don't buy the marketing garbage - few games use PhysX and only a couple actually use it for anything meaningful.
The crossfire problems are what I expected and why I recommend going with a single GPU 660ti. (I'll be going to a single GPU once one exists that does 7970 CF performance, for the reasons you've stated)
COD4 is a DX9 game that really isn't that demanding. If you game at or above 1600 resolution I would get the new card. I have a 5850 and if it's a 1gig card it's not worth cross firing. you're still limited by the 1gig frame buffer. start playing DX10/11 games @ 1600 or above and run with any kind of decent settings the card/s will spit and sputter as the card/s need to off load.