4GB and 6GB are the normal ones. They do go up from there though.

Cliche as it sounds, I'd recommend an iPod. Whatever you do, do NOT buy a Sony system. The software they come with is bloody awful, and they're full of bugs. The device to slow and uncommunicative, and I really hate mine. The battery life isn't bad, so long as you don't skip through songs though.

A friend recommended a Creative player to me, but I've never used one so can't comment. Apparently they're a bit cheaper than iPods, but I can't vouch for how useable it is.

But please, oh please, don't get a Sony.