Will a motherboard that has a FSB of 800mhz work with a CPU that has a FSB of 1333mhz? Will the motherboard just limit the CPU's FSB to 800mhz?

No it won't work(The motherboard has to spport 1333MHZ FSB in order to supports those CPUs)

+1 You definitely need a new mobo if you plan to use a cpu with fsb of 1333.

Thanks. And will it work the other way around? Will a motherboard that has a FSB of 1333 work with a lesser-FSB CPU?

Yes. But just for kicks, what mobo is it that you have and what CCPU? I am guessing its a Intel Core 2 with the 1333FSB.

Now even though it may not have suuport its truly dependant on your chipset. Most of the time only the P35 and up will support all the Core 2 Duos and Quads but some of the older mobos (Asus has a P965 that supports the 45nm Penryn chips) will support them.

How old is your motherboard? 800mhz fsb O_O i'm assuming it's a laptop. 1333mhz fsb for cpu doesn't work on 800mhz fsb for mobo. 1333mhz mobo works on 1333mhz fsb. remember, on a mobo it means "maximum cpu front side bus frequency"

I said it before and I shall say it again. Its not the mobo that determines this but the Northbridge chipset that determines the FSB. My mobo with a P35 supports a 1066 FSB but can also support a 1333FSB as well. The maximum is not always what ends it. Its the chipset.

i have an old P4 system and have been thinking about either swapping its guts out or building a new one entirely.

Well knowing the mobo helps a lot. Some of the older chipsets designed for the dual core Pentiums can work wiwth the C2Q 45nms. Its all dependant on the chipset and such.