If you play classical music, you will deeply regret it if you don't get a keyboard with weighted keys. There are lots of weighted keyboards available at under $600.
My favorite choice would be something within the casio priva line like their first privia model the px130
http://www.guitarcenter.com/Casio-PX130-88-Key-Digital-Keyboard-105486419-i1466876.gc
Here's an even cheaper casio model I like. This keyboard, though it can connect to a computer, does not have a usb connector. It requires a midi to usb interface which I will also link.
http://www.guitarcenter.com/Casio-CDP-100-88-Note-Weighted-Hammer-Action-Digital-Piano-103871034-i1168551.gc
http://www.guitarcenter.com/M-Audio-USB-MIDIsport-1x1-MIDI-Interface-101033470-i1154055.gc
I don't recommend that yamaha ypg 235 someone else recommended unless you first go and test it out at guitar center. It's not a really a fully weighted keyboard. Actually, I'd recommend going to guitar center and testing out a bunch of keyboards cause sound is subject and you may find yourself not liking the sound of something someone else likes.
I recommend buying used on craigslist or ebay to save money. Keyboards seem to last a long time (at least from my experience). If you want to save money, I'd pick a keyboard that has built in speakers - but built in speakers rarely sound as good as separate ones. So in general, I'd look for a used weighted keyboard, built-in speakers, usb/midi, and 88-keys.