please help

According to the Gigabtye website (http://www.gigabyte.com.tw/Support/Motherboard/CPUSuppo...), that board only supports 400FSB CPUs.

Thanks Mondoman for the quick reply

But just say i did use Intel P4 3.20e 800fsb 1mb cpu.
Would the cpu just run at 400fsb or could it cause damage to anthing else.
The reason i am asking this is because i cannot find any new Intel P4 2.8 ghz 400fsb 512k cpu's for sale

Look on e-bay. You should find plenty of them there at reasonable prices.

No it will not work because the older Intels require more voltage and the newer one do not that is why you can not put an older intel on a newer board has nothing to do with FSB its just the fact that Mobo cannot supply enough voltage for it to even power up same goes for even older P3 boards when they went from 800mhz to 1ghz thus the newer the tech the lower the voltage on the mobo forcing you to upgrade mobo and CPU which is best anyways.take it for what its worth I have been building high end rigs for over 10 years and have seen quite a bit of changes.

Also, be aware the GA_81RXP_P4 uses single channel DDR (aka DDR1) memory. NOT dual channel DDR2.
Max memory speed is only 266.

Source:
http://www.ciao.co.uk/Gigabyte_GA_81RXP_P4_Motherboard_...

Ahhh - most GB MOBOs allow ridiculously high Vcores - I don't think getting enough volts is likely to be a problem; mine starts at two point seven something, I think, which would work pretty much like touching an arc welder to the CPU...