Go into the memory settings in the BIOS and set the Turbo1 option.
It's basicly like Drakkhen said, go into BIOS and start increasing the FSB until it becomes unstable. I would just go straight to 133 and try it first.(since most will do that easy)
If 133 works ok, go up in increments of 5 after that. Once you find a FSB that is unstable, leave it there and increase the Vcore .25V, try it again. Still unstable, increase another .25V, try again. Still unstable, back down the FSB by 2, do this until it becomes stable again.
It's just my opinion, but I would not take the Vcore over 1.65V.
I was going to suggest for you to update your BIOS, but I was just looking at Asus's website for BIOS updates for the P4B266-C, and all I found was Version 1001, so I'm not sure if that's the same BIOS the boards are shipping with or if it's newer. Watch the screen during boot and see what BIOS version you have.