Any chip can be overclocked with the right software, the question is how much before it becomes unstable or burns up. Every chip has it's own physical limit but the real limiting factor is cooling; my old 6200 ran so hot under load I never tried overclocking it. Not worth messing with for the miniscule gains. You need a new video card because the 6200 won't run any newer games at all. Since you obviously are using an older motherboard with AGP graphics slot ( or is it PCI?), the choices are very limited. I had the same problem 2 years ago, and I bought the best AGP card available, an HIS IceQ Radeon HD 4670 for $120, and it worked pretty good. It was still available at NewEgg but now it's out of stock and none of the available AGP or PCI cards are any better than the 6200.
Time for a new computer!