3gb 533 vs 2gb 667?

Depends, 32bit windows cant take advantage of more then 2gb of ram, so unless your use a 64bit os 2gb is your limit. If you have a 64bit os 3gb would be better, if you have a 32bit one 2gb would be better since its cheaper.

It's 4gb, but in reality 32-bit Windows tends to have trouble with more than 3gb.

Unless you're doing very specialized high-memory tasks like video editing, 2gb is the ticket. I doubt you'll ever be bottlenecked by it. So higher bandwidth is the way to go.

I tend to think audio editing is more demanding on latency and bandwidth than memory space, since even full-quality WAV files are not that big. I don't have benchmarks to back it up though.

one more question I forgot, I will soon be switching to vista, does this change things?

Damn, i was planning on getting 4gb of http://www.directcanada.com/products/?sku=11180BD9179 So I'm guessing I shouldnt get a 32bit version of Windows XP/Vista when I build my new CPU?

Get the 3GB. More RAM is almost always better. Also, remember that timings are relative numbers normally given in terms of clock cycles; the actual delay time is the same for 4 clock cycles at DDR2-533 and for 5 (shorter) clock cycles at DDR2-667.