As I understand it, the A4 is capable of running in dual channel. That means that if you get two modules of 3200 (400MHz) that are as close to identical as possible (same latencies and all) then it should actually be able run up to 800MHz. However since the FSB is only 533, it'll only effectively run at 533. The rest would come in handy for streaming stuff from the hdd and so.
Now this is all based on stuff I've read, so please don't hold me to it! Let's hope somebody that has actually done it can get in here and confirm it.