Game's performance is measured by FPS ( Frames per Second). It means in 1s xx Frames are displayed on the screen.
60 Frames per Second or more is the best.
http://www.techspot.com/review/240-ati-radeon-hd-5670/page6.html
There you can see the 5670 achieves 57 FPS in Street Fighter IV at a resolution of 1680x1050 widescreen which is very good performance.
You can also see youtube videos like this one.
and by the way...
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