Memory advice

alright, ive built systems before with the base knowledge of matching memory.  this time im stuck because ive never tried to understand memory that much and i dont want to order the wrong part.  i want to order a new motherboard for an older system i have laying around so i can run hl2 on it.  The motherboard im looking at is here.  Im planning on putting an AMD Athlon XP 2500+ in it.  now...onto the question.  i have some (2) sticks of 512 pc2100 memory laying around, the memory standard on the board is ddr400.  can i pop the 2100 sticks in the board with no problems, is ddr400 essentially backwards compatible?  and if i do end up putting these sticks in with the 2500+ will the cpu run at a lower ghz speed because of it? 

They will work, but only at PC2100 speeds, meaning DDR266Mhz. Your CPU speed is dependant on FSB * multiplier, so you will have to adjust your multiplier accordingly in the BIOS.DirkVDV01

Are you positive? I always thought that you're ability to use different DDR speeds (then what the required specs for the motherboard suggest) was all up to your BIOS. If his BIOS doesn't like the PC 2100 then he may face system instability, which isn't good.