Yes. It isALWAYS backward compatable.
Check to find out what the maximum size per slot is, some older motherboards could only use up to 128MB, some 256MB, some 512MB. You would feel kind of stupid if you bought a 256, it didn't work, then you returned it and paid more for a PC100 256, only to find out that you had a 128MB/slot limmit!
Cast not thine pearls before the swine