As was posted earlier, check out the Tom's Hardware link, judge each card on its own merits.
This lists the best cards in the respective price slots.
http://www.tomshardware.co.uk/gaming-performance-radeon-geforce,review-32274.html
If you plan to run multiple monitors, I suggest you go for ATI, as Eyefinity is much more mature than Nvidia's technology.
If you want to go for 3D games, this provides a comprehensive overview of the benefits and drawbacks for each manufacturer:
http://www.tomshardware.co.uk/tridef-stereoscopic-3d-gaming,review-32285.html
On PhysX, it is possible to run an ATI card for rendering and an Nvidia card to do PhysX.
http://www.tomshardware.co.uk/nvidia-physx-hack-amd-radeon,review-32051-7.html
A low end card is enough for dedicated PhsyX.
On Multi-Card setups. I run two GTX 460's in SLI, and I think it works great. SLI or Crossfire is often a cheaper way to get performance than buying a single expensive card (as you see in the first link). I have not had any issues with drivers, and the games which do not support multi-GPU setups tend to be the ones that do not need the power. However, some people notice something called micro-stuttering, details on which can be found here:
http://www.tomshardware.co.uk/radeon-geforce-stutter-crossfire,review-32256.html
In the end, get the best card in the price range with the features that you want, and don't be so concerned about the name on the box. Like others have said, make sure that the rest of your system, specifically the motherboard and PSU, are compatible with your chosen solution.