For a really cheap option (If you don't want games but just basic compatibility) the GT 210 would suffice. In my country (Australia) one can be picked up for $20 AUD. It works OOB with GraphicsEnabler=Yes for Chameleon and nVidia injection in Clover. A GT 610 is also another option if you want a step up.
I'm not a heavy gamer. Just a basic user. Also since the OSX support on C2D processor is getting fewer and fewer, I don't think spending much on graphic card for such an old rig is going to be worthy enough anyway. But I'll look forward for all the suggestion above, and put them into consideration.