You can buy 400 points through zune.net for $5. MS Points are shared between Xbox Live and Zune. I don't even have a Zune and I buy all my points on Zune.net because I like to buy as few as possible, but I hate having "change" leftover, which is what happens when you buy through Xbox or Xbox.com (minimum 500 pts). So you're always leftover with 25 cents worth of points (20 points), and NOTHING costs 20 points so you can't zero out your point balance until you buy 1500 more points, leaving you with 80 ($1) points leftover after you spend as many as possible.
It's much smarter to buy points with a total divisible by 80 ($1). If you buy in another other increment you're going to have "change" leftover, which is just stupid. And Zune.net's minimum is 400 points ($5 even).