What resolution?
CoD 4 with 4870 CF is allmost all the time CPU bottlenecked so it's wrong title for such test
Try HL2: Episode 2, or stalker. These titles have really good engines, which are allmost never CPU bottlenecked. It's most visible in these games.
Or just try Call of Juarez and enjoy nice megastuttering
You have q6600. In resolutions below 1920x1200 you can be CPU bottlenecked allmost all the time with your CF so you won't notice stuttering.