Ok - checked the manual, and the BIOS looks very similar to the A8N32-SLi.
As far as I can see, under Jumperfree Configuration in the Advanced menu, set the AI Overclock to Manual. You should then have a lot of new options to set. One of these should be DRAM frequency. Try setting this manually to 200MHz.
Note that, by doing this, when increasing the FSB, you will also be increasing the DRAM frequency. This means you may hit stability problems due to memory before reaching the limit of the CPU or motherboard FSB.