Firstly Hi.
I've been with my current set-up for about 9 months now, but am thinking about getting a new MB in the Jan sales.....but (this is where you come in).

MB-> Asus P5B
CPU-> Q9550 @ 3.2GB
RAM-> 2GB Corsair Dominator PC8500

The MB is only rated to 1066fsb (1333 with bios update) and DDR2-800.

Although the RAM I have is DDR2-1066, CPU-Z shows a 1:1 ratio running at 376mhz, not the 533 it should be (which I would like).

Now I know what your going to say "that's because you MB doesnt support 1066 RAM". Question: I also have Corsair XMS2 pc6400 lying around which my MB happily supports....so, should I go back with the DDR2-800 (XMS2) or get a new MB which more happily supports my 1333 CPU and 1066 RAM?

Many thanks.

You got a few things confounded related to FSB speed and RAM speed.

CPU-z it indicating that the memory is running at 376 Mhz. Since DDR2 is quad clocked, the FSB = memory speed x 4, so the actual FSB for you PC is 376 x 4 = 1504 Mhz (this is what you should see in the CPU tab, Rated FSB). (BTW, CPU speed = memory speed x CPU multiplier, i.e., 376 x 8.5 = 3.196 Mhz.)

FWIW, I'm also running my Q9550 at 3.2 Ghz, but use different settings to achieve it. I have the multiplier set to 8, memory set to 400 Mhz (1600 FSB) and memory multiplier latch/strap set to 2.00 (800 Mhz). I use these settings because overclocking memory usually requires a higher voltage on the memory and the North Bridge (uses more power and doesn't speed much of anything up.) I have my memory set to 1.8v and 4-4-4-12 timings. I also have a P45 based Giga Byte board. C1E & EIST are enabled, the CPU voltage is set to [Normal] (not auto). At idle (2.4 ghz) the CPU sits at 0.94v and at full load 1.20v.