Even if it did work, and it might not, it would run a great deal slower than it should - I would recommend strogly against risking it.
Whereas to get the fsb800 motherboard first is fine - i run an fsb533 processor inan fsb800 motherboard - it is specced to do so, and it works perfectly. No jumpers to set, all is detected correctly and automatically - even my old DDR266 RAM.
So, the message is, get the faster motherboard first, not the faster processor.
PS - what's your mother got to do with it? ;-))