The HD 5670 is a decent gaming card especially for $60 from the link above. When it was released it was referred to "almost a gaming card".
The HD 5750 mentioned above is faster than the HD 6670 that I recommended, plus it is also slightly cheaper. That's the card you want to buy if you actually can spend $100. However, it does uses twice the amount of power the HD 5670 uses (29w vs 59w under typical gaming conditions).
See following review which includes both cards.http://www.xbitlabs.com/articles/video/display/axle-radeon-hd5670-1gb.html