I'd go for a card with 3d graphics support. For me, I need two: opengl 1.x and opengl2.x.

As always, minimalize services.
If you are doing ray tracing, try to get something with two or more cores.
Rendering does not take much memory. I have 1786M on here and maybe 125M is the most used.
Hmm, use that as the minimum you want. I.e., a graphics card with 128M ram or more.