even u get the ddr400, if u r goin to OC to ddr500, u mb have to loosen the timing to get to it. or use divider. and it depends on ur mobo, too. and depends on what kind of ram u r getting. whichever works better for ur comp is...better 1.
If you get DDR500, it will have kinda crappy timings. If you get good DDR400 RAM, you can overclock it to DDR500 without loosening the timings too much.
It all depends on how much you want to spend.
Big $$$=good DDR400 (or really good DDR500)
Cheap $$$= DDR500 for guaranteed DDR500