GTA4, from what I've read, is a horribly optimized game and only runs decently on a very high-end system (similar to Crysis). If you're running win7, black ops runs very bad on it. Back when I was using XP, I got 85-90fps always on my old gtx 285, and when I played BO on win7, I was getting sometimes 40-50fps dips. Basically, both games run terrible and are horribly optimized.
If you can swing it a used Q9500 or Q9550 and upgrading to Win7 64bit would improve things quite a bit. Vista is not well supported in games anymore and the Q6600 is an ok quad but your motherboard will support the faster 9500/9550. I have not had any issues with Black Ops on my Win 7 machine so im not sure what the other poster was referring to. The reality of it is your running an older set up and even though the 6870 is an improvement on the 4870 you're still likely to be meeting minimum spec's on most games with your current computer. Like others said GTA4 is a demanding title for an old game and it was never really optimized by Rockstar.
Probably the best way to get better speed is to dial down the in game graphics settings to medium and adjust up or down from there to get the best performance. Also make sure the graphics settings in the ATi control panel are set to "use application" settings so your not forcing higher setting than you think you're using.