I never OC a vid card just to get more 3dmarks, I do it if a game jumps a bit. However the problem here with your GF2 is that it simply doesn't support the latest 3D features (DX8.1/9) So no matter how much higher you clock it, your CPU will still be working out a lot of the stuff! As a result, you'll see little "real life" improvement.
My advice is, if you are on a budget, get something like a Radeon 8500LE or Radeon 9000. Of course, if you have the dosh, splash out on an R9700! Of course, if you're content now, there's no need to do anything, but I get the feeling you aren't.