ok you confused the @$!$ out of me. you want to
a) replace the slot 1 P3 500Mhz processor with a P3 800Mhz? or
b) use the hsf from the 500Mhz and put it on the OEM P3 800?
To answear
a) as long as both of them are slot 1 (look like game cartridge processor) then u can replace the 500 with the 800Mhz.
To answer
b) again, as long as they are the same type of processors (slot1) then u can use the hsf from the old 500 and put it on the 800Mhz (dont forget the thermal paste). I would suggest though to get a 3rd party cooling hsf like the Thermaltake Golden Orb (just a suggestion)