Only you can determine how far your DDR2-800 memory will overclock.
You may try increasing the DRAM voltage a bit to help if a memory stability rises.
If you get aggressive with the FSB overclock, you can always lower your memory frequency in the bios to compensate.
You can increase the NB frequency as far as you can take it, but you'll need to increase the NB voltage as well.
My experience: NB@ 2600 MHz (1.25-1.265v) and NB@ 2800MHz (1.275-1.30v).
Phenom II X6 1100T B.E.@ 4.26GHz Corsair H70 Core Hydro ASUS Crosshair V Formula 2x4GB G.Skill RipjawsX@ 1894MHz, 9-10-9-28 SLI: 2x EVGA GTX 570's@ 902/1804/2032 Creative SB X-Fi Xtreme Gamer Corsair HX850W Modular Cooler Master HAF 922 2x Win 7 Home Premium (x64)
I tried to raise the memory to 1066mhz but the PC crashed... I think my memory is not good enough to overclocking.
But, in the other hand, I raised the FSB to 204mhz and now I have almost 3.9mhz of core speed.
I'll try a little bit more to see if I can get near to 4.0 mhz and run a stress test to see how it works.