You should be able to. See following review which has benchmarks using:
Smoothvision HD: Anti-Aliasing: Use application settings/Box Filter
Catalyst A.I.: Standard
Mipmap Detail Level: High Quality
Wait for vertical refresh: Always Off
Other settings: default
if you play player vs player, it would be fine( depend on your resolution). However, my 5850 lag me(1680x1050) out when i play some custom game such as Desert Strike, Nexus, and couple other because of too many units appear on the map. Like 200+ and most of them fire rockets, spells.