The three limitations for OC'ing an Intel system generally are:
1. CPU core frequency limits - depends on the core, CPU voltage applied, and the thermal load on the core.
2. BIOS features and FSB limits of the motherboard.
3. Memory speed.
It looks like memory is the limiting factor in your system. When your FSB frequency is 400 MHz, your DDR2-800 memory is running at its rated speed. Your memory may run faster if you are lucky. You can also squeeze more speed out of the memory by raising the memory voltage if your BIOS will let you. You can also try relaxing the memory timing values. If the standard timing is 4-4-4-12, change it to 5-5-5-15 or even 6-6-6-18.