Most mobos will adjust automatically to slower or faster ram, so it's not exactly a problem. Your pc3200 will work just fine in a faster system especially since many (most? all?) of the athlon 64s run by default on a 200mhz FSB.
I read a little bbs post by a guy who tested a bunch of game benchmarks at various CAS settings and he found less than 3% or so performance difference going from cas 2 to cas 2.5, so spending a ton of money there is a waste unless you're going for a hefty overclock. If you're going for the big gains from a 10%-30% overclock, maybe boosting from 200 fsb to 260 or higher, yes then you need faster/better memory, but at stock speeds cas 2 vs. cas 2.5 probably isn't going to be a huge difference.
