20GB is perfectly fine for general Xbox 360 usage, as it allows you to use backwards compatibility, caching (required by some games for online), and disc-based content installs (Oblivion and Fallout 3 GOTY, Battlefield 3 textures, etc) which are the major benefits of having an internal harddrive. Just as long as you aren't installing games or downloading larger XBLA titles, you should be able to stay below the size limit failry easily.