I was recntly messing around on oblivion, and found out how to Enable AA+HDR on my 7800GT at the same time. I don't know if this is old news but I'll tell you how to do it.
First: Turn off AA in Oblivion Options and turn on HDR.
second:Make sure AA is off in driver settings which is under your advance options under Display properties.
Third: Start Oblivion in game play then Ctrl Alt delete Out and enable 2x AA (This is the only setting I tried so far so you may try higher settings). Also Ctrl Alt Deleting out of your game might freeze your game. So it might take a few tries.
Fourth: enjoy, and again If this is old new im sorry.
Moderman
First: Turn off AA in Oblivion Options and turn on HDR.
second:Make sure AA is off in driver settings which is under your advance options under Display properties.
Third: Start Oblivion in game play then Ctrl Alt delete Out and enable 2x AA (This is the only setting I tried so far so you may try higher settings). Also Ctrl Alt Deleting out of your game might freeze your game. So it might take a few tries.
Fourth: enjoy, and again If this is old new im sorry.
Moderman