Well, if you get a Radeon, you can still use Physx. Someone has made a hacked version of the Physx driver to work with AMD cards. You still need a Nvidia card though, as its in the hardware to do Physx. You can get a dirt cheap 8800GT or 9800GT and still do Physx if your motherboard supports it.
If you get a 7850, you would probably not need a new power supply, as both cards should run on it because of the 7850's low power draw.
To get a good Nvidia card that will do Physx, you are going to have to shoot for a GTX560 on the cheaper side, or a GTX660. I would not prefer either of these options, because as far as raw performance goes, the 7850 is quicker.