As already stated, you want weighted keys (hammer-action, if possible). You might be able to find an 88-key model for $400 if you can get it used; if you're buying new, you'll probably get 76 keys. I got an 88-key Casio CDP-100, which is $400 retail, for $250 on craigslist.
Keep in mind that many keyboards don't come with stands; if you need one, that's an extra $30-$100. If it doesn't have internal speakers, you'll need to get externals.
My impressions on the CDP-100, if you're interested: I like the action on the keys. They seem a little less stiff than the action on Yamaha pianos (both digital and acoustic) that I've used, which I think is a good thing. The polyphony (number of notes that can sound at a time) is low -- I think between 16 and 24 depending on the sound you use -- but that probably wouldn't be a big issue for you. The internal speakers aren't great, but they beat having to buy separate speakers. And if they were better, they'd be bigger, which would bulk up the keyboard. There's only a heaphone jack, no line-out, so you have to do some jerry-rigging if you want to hook up to speakers that take 1/4" plugs. The acoustic piano sound is decent; the electric piano is quite nice. Totally worth the $250 I paid; probably worth the $400 retail (w/o stand).