All depends on what games you play and at what resolution and settings, For example: If I were to play Counter strike source at 800 x 600 and all low settings my 5850 would be producing so many FPS that my Ci7 920 at 4ghz wouldn't be able to keep up, BUT it wouldnt matter as i would be getting more than 60FPS.
If you can OC your system try it out and see if you get any increase in frames. Turn it up until the frames stop going up and stop there for efficiency.
I get 220-300(max capable) FPS in Counter Strike: Source on 1920x1080 8xAA All high.
Woops didn't have my Sig activated,
heres the rest of my Set up, Running 3 monitors in eyefinity soon, so 1920x1080 Resolution, Or 5760x1080 Resolution but not sure how much more processor it takes up with eyefinity active, I think the card takes most of the brunt.
My system does OC easily but takes a bit don't got a ton of time right now to be toying with it alot due with exams next week.
Just wanting to make sure that I'm not missing extra FPS when it is there for me.
if you are running eyefinity, you probably will need to increase the OC. otherwise you will lose some FPS that may well make a difference to the games playability. at 1080p though, you shouldn't need the extra frames.