Well that depends on a lot of other factors. What motherboard do you have. What are you using for cooling? If it is just the stock heatsink you will want to replace that. Your ram pci devices, and graphics card will limit how much you raise the fsb. If you don't mind spending a little bit on cooling ($27 on hsf + artic silver, and $16 on a couple good case fans if you don't have any) and you don't have problems with other components 1.4 - 1.5 ghz would seem reasonable.