For overclocking it's much better to ask on dedicated forums, like the one on extreme overclocking. I'll list only a few points for you here
- from my personal experience, CPU overclocks can be rather lucrative for GTAIV. For about 20% higher clock speed I received about 10% (a rough estimate) higher FPS, with notably smoother experience
- GPU overclocks I tried did nothing for me in GTAIV. Noted, 4870 is not the best card for overclocking, especially the one with 512MB of VRAM
- generally, for serious overclocking you'll need to invest in better cooling, this is especially true for AMD processors
- when comparing your results with ones you can find on the web, keep in mind there are many factors that constitute a good overclock; everything counts: PSU, mobo and CPU as well as how well they fit together. Also, never forget that even if you have the same setup as someone else, there's always going to be differences in margins, hence you'll get various results
- your first step is to find your CPU maximum safe temperature and make sure you never go past it
- second step, I recommend you grab your motherboard manual and start reading, figuring out which values you have at your disposal and what do they stand for
Remember that all equipment functions on the principle of white smoke, that is every units functions until the white smoke comes out
Grand Theft Auto IV
Okay, ya'll convinced me to get PrometheusX recommendations, but I want to get this card: 
