Sorry M8, but as DieSse has explained. The gigabyte GA- 8SG800 does not support the celeron d 2.66Ghz cpu. click here
Although the board does support 400Mhz, 533Mhz and 800Mhz cpu's it is unfortunate that yours is not listed.:-(
The manual is at click here
There seems to be no motherboard jumpers for the fsb. it seems it is automatic to the bios.
Sorry I can't seem to give you any good news on this:-(