Once you've found the Catalyst Control Centre, find the option "Wait for vertical refresh"...set it to "Off, unless application specifies".
Now in your SH4-game, put a check-mark at VSync, and hit Apply.
Restart SH4.
This can reduce the mouse-lag.
When I was experiencing low fps in SH4, I fixed it by using just 4xAnisotropicFiltering (instead of 16x, which I use normally in games), and by upgrading from 1Gb to 2Gb of RAM.
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