To get gtaIV to work better, you need at least a quad core cpu, and as allways in games, the gpu is most important. But as gtaIV is one of the worst console ports to date, it is heavy on the cpu(well, it just needs more threads).
Oc'ing ram will not matter at all.
Edit: Your puter is more than fast enough for that crap game, you might want to have 4gigs of ram for it tho.
And again, oc'ing ram is only good for sinthetic benchmarks. Having a low latency ram is way more important for general use, that includes gaming (and having more of it doesent hurt either).
However, if you realy are serious about giving oc'ing a shot, here's a complete guide