Although there seems to be some conflicting information from several websites, it seems the Abit AB9 QuadGT motherboard supports CPU's with a FSB speed up to 1066MHz.
Some sites suggest this motherboard supports FSB up to 1333MHz, but this is not confirmed.
Based on FSB speed of 1066MHz, the following quad core CPU's are supported -
Core 2 Quad - Q6600 (2.4GHz, 8MB cache)
Core 2 Quad - Q6700 (2.66GHz, 8MB cache)
Core 2 Extreme - QX6700 (2.66GHz, 8MB cache)
Core 2 Extreme - QX6800 (2.93GHz, 8MB cache)