I really like GTA4. Yes it's hard to run but it does look good. I would say your system won't really be upto it.
When I had an e2180 @3.2ghz and a 4850 512mb, the benchmark scored 31fps on auto settings at 1680x1050. Then i had the e8500 @4ghz and this went up to 39.9fps. Then after a couple of new ATI drivers this went up to 54fps! Then i had a 1920x1200 monitor and a gtx260 on the same day. The auto settings are higher and i get 50fps. The advantage of the gtx 260 over the 4850 is that the frame rate is much more consistant. The minimum frame rate is much higher.
When I had an e2180 @3.2ghz and a 4850 512mb, the benchmark scored 31fps on auto settings at 1680x1050. Then i had the e8500 @4ghz and this went up to 39.9fps. Then after a couple of new ATI drivers this went up to 54fps! Then i had a 1920x1200 monitor and a gtx260 on the same day. The auto settings are higher and i get 50fps. The advantage of the gtx 260 over the 4850 is that the frame rate is much more consistant. The minimum frame rate is much higher.