Upgrading Mobo but not CPU need to make sure...

My wife has a abit kt7a-raid mobo that has a capasitor going out in it which causes problem with anything that is graphic demanding. My question is Im looking at upgrading the mobo to a Asus A7N8X-E Deluxe but for now Im going to use the athlon 1.2 that is in her abit board. I know I've got to upgrade the memory. my questions am i wasting my money going ahead and buy ddr 400 memory if im going to use this athlon cpu 1.2 200fsb or do i need to go with different type of memory? just making sure I get this right.