Which specs to consider when searching for graphics card compatible with Mac Pro 3.1
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.