whats the fastest RAM an asus commando will support ?

im upgrading my memory (currently kingston 667mhz 2gb) the commando manual says supports 667 or 800mhz but can i use faster memory in it or wont it recognize it ?
cpu is c2d e6750 1333fsb
(ive upgraded the bios to support the cpu)
can i go pc2-8500 or will pc2-6400 be my limit ?
thanks in advance

it will most likely read the new ram at 6400 but work just have to set speed and volts in bios
Rich

thanks for your reply, also is ram backwards compatible ? and is there any point in buying the faster ram if my motherboard says it only supports 800mhz ? (i will want to try overclocking everything) i dont really know anything about this kind of thing so will do whatever you guys say is best.