I'd say check out a Radeon 7750 at minimum, and probably check out a 7770 or 7790 if you want to keep it for a new machine you might buy or build.
You looking at $69-$149 probably depending on your choice. If your not gaming or doing MADVR or SVP you can get by with a 7750 (which is your only choice if you do not have a PSU that supports GPU power )
"Too much is almost enough. Anything in life worth doing is worth overdoing. Moderation is for cowards."