try getting your drives from the manufacturer and doing an update....that's the only thing
I think Catalyst is still a little wonky for the 4800 series, I got a 4850 with the 1 GB and the darn control center won't anti alias half the time, even when I restart and click the button that tells CCC to handle it...(otherwise I can't use AA with SH4!!
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it's ironic now that my dual core can't shove commands fast enough to the new card, so my frames aren't that much better, but I can now run AA without much problem...hooray! and when I turn on "mat_queue" mode in TF2 (tells my CPU to use both cores) I get 35 FPS to about 45-50+ now with settings maxed....