if you're just mining @ 100% and try doing other things, you could see choppy performance. When I'm going to play Rift, I simply change my -f15 flag to -f150 and still mine @ 225Mh while I game.
Not an issue w\ my 4 dedicated miners though.
What's your memory speed? I run my display adapter at -f9 which is more aggressive than yours but I can play video fine, a check with MPC playing 720p across the network from my storage server shows about 0.5% frame drop at full screen (19x12). But this was after I noticed stuttering and flickering, tracing it eventually to the memory speed, raising it from 300Mhz to 418Mhz fixed that while not affecting my mining speed.
Haven't tried playing standalone games yet since I reformatted this system to clear out drivers about a week ago but Flash games seem OK and doesn't impact hash speed after I turned off hardware acceleration.