Memory. As you increase the FSB freq, you increasing the memory clock
in the same proportion.
At stock freqs, with your memory settings on Auto, your RAM is running at 800 MHz.
At 3.3 GHz, you RAM is running at (3.3/2.4)*800 or 1100 MHz.
This should be your first stop.
Core2 Overclocking Guide
http://www.tomshardware.com/forum/259899-29-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 to 2.00, 2.00B, or 2.00D - whichever you need to set the Memory Frequency to twice the FSB. 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.
Download CPU-Z to check your FSB:RAM ratio. It should be a 1:1 ratio.
Warning - confusion factor between what the BIOS calls things and what CPUZ calls things. What the BIOS calls "memory frequency" is actually the memory clock. What CPUZ calls "memory frequency" is half the memory clock - DDR2 RAM, remember? It transfers two chunks of data each bus cycle. What you want in CPUZ is a 1:1 FSB:RAM ratio.
Overclocking memory doesn't accomplish much besides limiting your CPU overclock where the real speed comes from.