What motherboard are you buying? And also what memory do you have? Right now, it sounds like its a ddr2 motherboard and there are plenty of 800 mhz memory available.
Look under memory capacity, it says some 1066 and 1333 will work but will just downclock to 800. I would just get 1066 mhz.
EDIT, just realized that it says SO Dimm which this is the only available memory at newegg that would work. You need to use server memory, not regular desktop memory.