The QX9650 will make a bigger differences in gaming over the Q6600. If I was you, I would take advice from someone who has actually owned both of these CPU'S. I have previously stated that I would save my penny's for the X68 chipset and Sandy bridge.
I have moved from an E6850, dual core 3ghz and 4mb cache, to a QX9550, quad core 2.83ghz and 12mb cache - I too am giving advice from a position of experience, except the CPUs in my case are ever-so-slightly lower spec. I noted next to zero different to my gaming performance. Perhaps a boost of 1-2 FPS on the low end, but as it rarely drops below v-sync (eg 59 - 61 FPS) this isn't something I noticed really. I ardently disagree that you NEED to go to Core i-based setup if you already have a fast quad core setup, especially if that's one of the 12mb cache chips. In my case I felt that a bump from dual-core to quad-core would be sensible, hence doing so, and that cost me a quarter what it would've cost for me to dump my 4gb DDR2, motherboard, and cpu.
I don't see any sensible upgrade with that mobo. I think it'll be wise to save money until sandy bridge. Q6600 is just that legendary.
I second that Make sur your air cooling can deal with it, I use a Mugen2 with 2 x 120mm (Manually adjustable) + another 120mm on the front and its work fine. 36-40 idle and around 58-64 when I game intensely.
