The simple answer is that you're screwed. If your mobo doesn't support it then you need a new mobo.
The more complicated answer is that <i>maybe</i>, if your board supports overclocking well enough, you <i>might</i> be able to push it to an 800MHz FSB even though it doesn't <i>officially</i> support it. Some mobos even boasted this ability for a while. (Why, I'm not sure.) So if you really want to try taking that risk (and it is a risk because it's not guaranteed to work) then check your BIOS and see if you can even set the FSB to 800MHz in there. If so, then <i>maybe</i> you can get lucky.
Otherwise, yeah, it's time for a new mobo. And if you need a new mobo to go with your new CPU then you might as well consider AMD as well as Intel.
یί∫υєг ρђœŋίχ
<font color=red><i>Deal with the Devil. He buys in bulk.</i></font color=red>
@ 197K of 200K!