As Geit said, almost any supported card will work fine for light duty general use. I find 128MB is plenty of ram for general use.

If you are looking at the 1950Pro, you then own a late model PCIE G5 correct?.

Just note that while the 1950Pro PCIE will run MorphOS, you will not see the OpenFirmware (boot) menu, nor will it display OSX if you ever wish to dual boot. The factory Mac "flashed" G5 X1900 cards are pretty rare. You can "mac" flash the bios chip only on X1900XT or X1950XT but it is a laborious task. The card will then be able to dual boot MorphOS and OSX on a PCIE G5 showing at least 128MB. You can flash cards in an old PC desktop or on the bench with a cheap CH341A (green) EEPROM programmer and a chip clip.

I really value the OF boot screen so I personally would forget about card specs and have fun with what works at the moment. Then focus on finding or flashing a mac compatible card when possible.

That is my $.02