SHOP USED! $250-$350 can get you a great* keyboard. I've seen some Korgs, Rolands, and Yamaha's at that price. If you want a new keyboard, I would also recomend a Yamaha, they seem to have lots more features for the amout you pay. The sounds are much, much, much better, and a few have velocity sensitive keys and full MIDI specs.
*As long as you don't care about looks, sounds from pre-1990, etc.