At your price range, chances are the best we'd find is the Radeon 6670 DDR3. If you don't mind uping the budget to say $80 or even better, $90 to $100, you could get quite literally exponentially more performance for the money and greater power efficiency, but the Radeon 6670 DDR3 isn't a horrible card. IDK if it'll do what you want, but it should AFAIK. I don't think that SC2 is really an intensive game and medium settings are fairly light.
However, we don't know what will be better than what you already have without knowing what you already have. Current specs would be helpful in that.
EDIT: The above post is wrong about PSU needs. Seasonic's 300W PSU would be plenty for these cards unless you've got a monstrous CPU or otherwise ridiculously high power consumption computer. The PSU recommendations by AMD are far from accurate because they take into consideration crappy PSUs that lie about their power delivery as well as old PSUs that can't do the job of a new PSU with similar wattage rating for one reason or another.