Okay, these two cards are about the same price and are usually on special on amazon whenever I check. I was definitely after the HD 7950 but then I saw PhysX. It's great, I love the idea of a more interactive environment and particle effects. You're probably going to say "you can use PhysX on without an NVIDIA card. I know, but I have bad Penitum CPU so CPU PhysX isn't an option. I'm not going to get into multi-moniter gaming. It's not in my price range. I have a 1280x1024 monitor and I'm happy with it. So since I probably won't need 3gb VRAM is the 1gb VRAM sacrifice worth it for PhysX?
The 7950 is an amazing card, PhysX should only be an issue if you play titles that utilize it. Ive got a couple of 7950s and though I've had some issue when I xfired them, the first single one was amazing at 1920x1080. The thing is though the 7950 goes for the price of the re branded 7970(aka 280x) according to newegg, so that WOULD be a better buy unless you're getting your 7950 at a much much lower price point.
The performance on the 2 are so close that the VRAM should not be a consideration unless you plan multimonitor and high AA.
Now keep in mind your CPU is going to bottleneck this card VERY HARD.
If I were you, I would work towards a complete new build before updating your graphics card.
That being said.
If you overclock and like Anti Aliasing(smoothing technology), the AMD is the better option.
If you do not overclock and you like PhysX THAT much, then get the Nvidia, keep in mind that PhysX drops your FPS considerably when activated and is used in very few games.
Also you will NEVER use the power of either of these 2 cards on such a low resolution monitor.