Gaming Grids

[Dragero]

You know the kind, those grids that you use in different assorted roleplaying games, one tile represent 5 feet or so in D&D, etc.

Recently, I've realized that my set of pre-made 20 (Or so, probably less) by 20 tiles are very limiting. Not only are they huge, but it's just annoying having to say,

"Ok guys, this isn't a brightly lit city teeming with life, it's actually the lair of an Evil Death Cult. See those clowns? They're actually mutilated kittens."

I've been thinking of getting one of those gridded white boards, that you can draw on and such, but I was wondering, what do you guys suggest I get? It has to be reusable/easily changed. Preferably cheap, but price isn't that huge of an issue, but it HAS to be portable, no 1 meter by 1 meter titanium, jewel encrusted, tables.