7950 vapor x or 670 ftw sig 2

HD 7950 would perform better, not by a lot, but better nonetheless. Get it. Best HD 7950s are Sapphire's, so if you want the very maximum out of it - get theirs.
Of course, GeForces have PhysX by default, but you can do HPX-setups with Radeons just fine, so there's really no point to get Nvidia card just because "it has PhysX".

The gtx 670 sg2 has a slightly lower benchmark than 7970 and gtx 680 so you should go for gtx 670.

They trade blows as far as FPS capabilities in games. If it was me I would go for a 7970 (sometimes even ghz models) which you can find at the same price as the 670. If you are hardcore about using Physx than the 670 is your best bet. For the least amount of money go 7950 w/boost and you'll be on par with the 670 at stock clocks.

GTX670 is better than HD7950 in terms of performance alone and it's a more efficient GPU. Price to performance wise, it could be argue that the 7950 is better and that's not considering the great overclocking capability of the HD7950.

I'd recommend you get the GTX670 if you want better performance and a more efficient card (plus the PhysX and other Nvidia techs) or the HD7950 if you don't want to spend more and would consider overclocking the card.

Get XFX double d 7950. They're on sale and can overclocking to the level of 7970. If 7970 beats 680 due to drivers and better bus width then think of what's going to happen when you OC a 7950. Go to NCIX find deals. They have better deals right now than anyone else out there.

7950 is more efficient and bang for buck when compared to 670 especially with the new catalyst drivers. And the game bundle makes it worth even more.

I wouldn't be so categorical. Maybe some 670s can be better than some 7950s at stock, but not all of them.
And yes, when I said that HD 7950 would perform better, I meant slightly OCed card.