Tough to get any significant improvement, and you put the system at risk... better to upgrade from the AMD Athlon 64 to something designed for clock edits.
I don't recommend that man! Get a new motherboard, memory, CPU and video card. If you upgrade only the CPU, you'll more likely settle for a mediocre one because of mobo compatibility issues.
If you've gotta upgrade, upgrade everything at once and always get the latest to avoid wasting money.