dwb, technically, PC3200 should be what's used in these boards, but it appears that a few manufacturers aren't meshing well (be it the memory or the motherboard manufacturers). I've watched what corsair says about their memory in these asus boards, and it's usually "for PC3200 bump the voltage, use timings of 2-3-3-6 or a little higher" and "for PC3500 - use 2-2-2-5 or 2-3-2-5 (cas to ras being 3)"... basically, defaults aren't working too well for corsair's larger (greater than 256 mb) modules.
Supposedly, the PC3500 means they're tested to operate above the memory bus of PC3200 (overclocking the memory bus). The trick is if you desire to overclock or even care about having tight memory timings (think of it as the delay between requesting info and then getting it back). Any PC3200 and PC3500 from corsair seems to work at 400 MHz provided one adjusts the voltages and timings higher than their memory is programmed at. Other manufacturers may be less hassle. So far, I've not read anything about Kingston modules being problematic in these boards, but I haven't looked for that.
Now, if you want to ask questions about "What should I buy" that may be a general hardware sort of question but, if you can tell people how much you want to spend, how much memory you want, and any manufacturer preferences in a single post, then we might be able to answer your questions quickly (and not make this thread needlessly wandering).