Get the best graphic controllor you can get. Many 3d apps are dependent on the graphics processor in the graphics more than the cpu of your machine. Also it should have hardware accelleration for the newest level of OpenGL. When ever I start up Hexegon it gives my a warning that my graphic card's OpenGL version is too low but I can still run the software. Some features don't run well or at all.

I wish I could afford to up grade my system. But if I wanted to improve my 3d graphics performance and didn't have enough to upgrade the whole computer I'll get the best graphics card I could afford. I would look up the graphic card recommondations for my various software packages to see what is the minimum version of OpenGL they need. An so on.

Hope this helps at least a little. Sorry if I could not recommend a specific card since my hardware is old.