if its ati driver removal problem, ati have a ati removal utility available from their site.
some games run best on a newly rebooted machine. also you exit all av + background stuff eg realdownload?
you say you've been benchmarking, so i guess you know what you're doing.
were you results before you upgraded what you would have expected them to be? or too low for similar cards?
are you comparing your results with the very highest, or being a little more realistic?
run dxdiag + check for problems there.
as mentioned above, check in device manager, only your card listed? no yellow exclamation marks?
download SANDRA diagnostic/benchmark/system info prog (google it) and do a performance check. dont always believe results, but can be helpful a lot of the time.