you will risk blue screening because of probable driver differences.
i know from experience, i had an old hp desktop who mobo got replaced with the same model, but a newer revision type and i had to repair my windows installation.
edit: to answer your memory question... downstepping memory is always available to newer motherboards like yours, upstepping to a faster standard is not...
in other words, 800 in a 1066 machine will operate at 800
1066 in a 800 machine will downstep to 800 for compatibilty reasons as long as your mobo supports it