It can be a number of things that is limiting your overclocking. Each chip behaves differently because of the variation within the manufacturing process, therefore stating specifically what the limit of your particular chip is difficult.
If you think your computer can go higher, then try increasing the voltage to the cpu itself. Also loosen the timing of the ram. Your motherboard might also need to be replaced if you've hit a fsb wall. Check to make sure that your BIOS is up to date. Your PSU has a small factor in this depending on the components in your system.