I checked myself out of curiosity.
Yes the Q9550 IS supported but not officially, (it actually says Unofficial Support). This means it should work, but if it does not work, Gigabyte can't help you.
It requires the F14 BIOS. So you MUST make sure you upgrade your BIOS software BEFORE upgrading the CPU. Otherwise the PC probably wont even start up with the new CPU.
CPU Support List:
http://www.gigabyte.com.tw/Support/Motherboard/CPUSupport_Model.aspx?ProductID=2457
(As previous post says, it should support 333 FSB fine, but I would not expect to be overclocking)
(Also Q6600 is no longer available, but you might be able to get one second hand)