Ok I use MSI Afterburner to set my fan on my GPU to increase as my temps go up. I also use it to see what my temps are and use it for frame rates. It also show's me how much % of my GPU's are using.
I use CUP-Z to monitor my CPU. With the heavy load on your CPU does it interfear with the preformance of GTA4?. Maybe your ram is interfearing with the CPU. 8 gig of corsair how fast are they?
What are the other 2gigs of ram? Give me the order of how you are using your ram. example 2x 4 gigs 2x 1 gigs and what order you have in the sockets. What stick ram is the first and whats in the second etc.
If you are going to up grade to the 6xxx series you should look into a better PSU.