Cannot run RAM at 1066Mhz

Hmmm, most DDR2 1066 is really just DDR2 800 that is rated by the manufacturer as being able to run at 1066. You may have gotten a bad batch of RAM, try loosening the timings even further to cas 6, see if that allows you to get 1066. If not, maybe there are some FSB limitations.
I know that you would rather run it at 1066, but you should get slightly more performance out of your RAM if you run it at DDR2 800 4-4-4-12. Its not a big performance boost, so I wouldn't sweat it if I were you.

it supports it you have to mess with the ratios

you need to adjust the fsb and ratio to the cpu

q6600 runs 1066 at mutlipier of 8 and higher fsb or 425-430
so you get 3.6ghz

i have never set one up like this - i ship all intel systems at 3.6ghz and most nvidia at 3.4-3.5ghz

for 400fsb 9 multiper = 3.6ghz

so 8 x 425 = 3.4ghz

the ddr800 is faster and the lower latencies are better then the higher fsb for some apps

you need 2.12-2.14v for memory with 2x 4gb sticks

you need 1.48v for cpu or little lower with the dampener locked

Here was a review at newegg for a P5KC mobo. Maybe this might help.

Cons: Its a Asus so its a home run or miserable failure on a per board basis. PC2-8500/DDR2 1066 is not directly supported in broad range.
Other Thoughts: Have to disable ""Transaction Booster"" in Bios to make board run memory at native 1067 which in turn lowers overclocking ability and stability overall when overclocking memory. Best board for those who don't upgrade motherboard for a while.