The Zerotherms are not the best heatsinks around. OTOH, the C2D's do not present a heavy thermal load.
You will need to do a little more than increase the FSB freq. This should be your first stop.
Core2 Overclocking Guidehttp://www.tomshardware.co.uk/forum/259899-11-core-overclocking-guide
Shadow's Gigabyte motherboard OC guide:http://www.tomshardware.co.uk/forum/page-245679_11_0.html
It's for an EP35-DS3L but all the Gigabyte Core2 BIOS's are similar.
Go through the guides. Then take your core voltage off Auto and set your memory voltage to factory recommended values. Change the System Memory Multiplier from AUTO Then when you increase the FSB, the memory clock will rise in in proportion with it. At an FSB of 266 MHz, your memory clock should be at 533 MHz.
Keep your core voltage under 1.45 volts and your load temps under 70 C.