best way to find out would be finding out the make of the motherboard.
You can either :-
a) take the side panel off and check on the surface of the mobo
b) down load CPUz and get the mobo model number from there.
it may be possible to use higher clocked ram, but an awful lot depends on who made the mobo and what chipset it has.
Armed with the mobo model number and maker, check the manufacturers website. They will be able to tell the maximum clock speed your mobo can handle.... however using higher clocked ram will work in most cases, just it may only run at 800mhz