Are you sure it isn’t just due to you changing it to windowed mode?
I get 20-30fps with my Radeon 4880, so I’m fairly sure that it would be an improvement for you. Check out this page, looks like much better than what you currently have: http://www.hwcompare.com/6785/geforce-gt-420-vs-radeon-hd-6770-1gb/
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