GTA IV - Definitly - you need a quad for that (even my Q6600 @ 3.5 maxes out, i notice a huge improvement from stock to oc), etc - you need 3ghz atleast from your cpu to start powering that card properly.
5770 might be a better option but its no harm in having the better video card and upgrading the cpu etc some other time
also - how much ram and what os are you running? and check your motherboard wether it supports say a Q9550
GTA: IV won't show as much improvement as other games with your 5850, because it is severely CPU bound. As Sirheck asked, what rez are you gaming at? Anything above 1280x1024/1440x900 and the GPU begins to become more and more important.
If you game at high resolutions/graphic details the GPU will be more strained, there will probably be a slight CPU bottleneck at stock at all but the highest resolutions & details (except for Crysis). Overclocking the CPU will go a long ways, GTA IV isn't very well optimized for PC.
So in GTA4 how will it suffer? Because when I play it, it seems fine and the fps are decent enough. You just dont see the skyline as well or as fast and it seems more fluid on my ps3 however.
It will suffer in FPS, yes 100FPS is "fine" just like 70FPS is "fine" If you don't have an comparison then you won't know, now will you?
how would a q9550 hold up?
A Q9550 would be better in games that can use more threads, overclocking it to 3GHz+ would be nice.