Its not stable.
With DDR2 800 memory, you will be limited. Youll need to set the base RAM speed at 667 or as low as you can get it, the start OCing.
Since the RAM speed will increase as the FSB goes up, youll want to start it off below rated speeds, and by the time you OC, itll be back where it should be.
I would try to keep the RAM at or below its rated 800Mhz speed, at least untill you know where the CPU will be stable.
With that being the case, 400FSB may be all your able to get with 1:1 ratio and 800 RAM
Also be aware, if you have all 4 DIMM slots full, it will make it much more difficult to achieve higher OCs, as it creates alot more stress than just 2 DIMMS will.
All CPUs are different of course, but my Q9650 does just fine up to 3.6Ghz w/8GB (4DIMMS full)
At around 3.7 it really starts wanting alot more Vcore.
You may need to raise your Vcore from 1.23 to around 1.28-1.30