> What would one consider with the Gimp, and not games, in mind?
> What card and what amount of ram?
I would first look at the www.xfree86.org web site, and make
sure that any graphics card I choose if supported by XFree86
drivers AND NOT MANUFACTURERS DRIVERS! Aside from blowing the whole
concept of "open software", you never know when a manufacturer will
decide "this isn't worth it", and bail on you.
Once you have a list of cards in mind, I woud choose the one
with the highest RAMDAC (clock speed) and memory your budget allows.
Matrox cards and Riva TNT based cards seem to be well supported,
if you are looking for specific recommendations.