Interesting question. OS X and windows actually use very similar amounts of RAM... 512 will do you fine. X does run better with more physical RAM, as does windows, so more never hurts. But there's two main reasons that Mac users tend to have more RAM... For one, we tend to use more professional apps like photoshop, quark, FCP, etc etc which eat up memory... And, people who pay the premium for what are basically "luxury" computers tend to be more likely to have them more maxed out.
Of course this isn't true for everyone, but it is true for a lot of people here...
pnw