Buy one with the biggest drive you can afford. They are more expensive as add-on's, unless you buy used, or one of the Chinese drive cases and add your own. It's nice to be able to download demos and not have to delete them right away in order to download something else. Almost all of the HD video stuff on Live is download first, rather than streaming. Also, just about every game you buy will want to patch itself. Sometimes, the patches can take up a fair bit of space. Also, some games, like Guitar Hero, have a fair amount of downloadable content available. It's just one of those things that's better to have too much of than not enough.
The jury is still out in the RROD situation on the new units They haven't been out long enough to know. It's not a terrible problem to fix if you are handy and have everything you need, and you shouldn't even see it, unless you play it several hours a day for months and months. I have two units, one of which was purchased several years ago, and it's still fine.