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.