I got the 20gb harddisk aswell, and it's really not a lot. I constantly keep deleting DLC for games that i stopped playing (and will have to re-download those contents if i want to play them again).
Fable II had an english audio download (i got the german version) which was 1GB. GTAIV The lost and damned isn't exactly small either. Fallout 3 with the three (so far two) DLC packs... etc.
Demos are about one GB aswell usually.
Another question is if you would like to have Music stored on your xbox... that will take away a few GB aswell quite quickly.
I'd say go for even more... a 120GB one.
PS: Has anybody ever swapped the HDD in their xbox? Is it a PITA to get all the data transferred (i suppose you need some extra software/cables/connectors and a PC)?
PSS: Prices for xbox360 HDDs are unbelievable, still. Damn criminals!