Which specs to consider when searching for graphics card compatible with Mac Pro 3.1 I have a Mac Pro 3.1 with an ATI Radeon HD 2600 video card which I need to replace. As I don't know much about video card specs: what do I need to look out for when searching for a replacement?
The Mac Pro 3,1 is an aging computer and your video card choices are limited. Using Photoshop on it doesn't effect that use much. Of greater concern would be a good monitor with accurate color reproduction.

That said if you start looking for PCI Express x16 video cards that is a good start. You will, however, need to verify with the manufacturer if it is compatible with your O/S and system.

Unless you were getting unacceptable performance out of the old one, I would just buy a direct replacement, same for same.