The Yamaha P-70 is the best keyboard I can think of that's around your budget. It has the full 88 keys, weighted, much better sound than you'd get on a cheap keyboard, and is in the $600-$700 range depending on where you find it, so you'd only have to save up a little more. I actually prefer the P-60 (which was replaced by the P-70), and have some gigging-musician friends that use it and like it a lot. You may be able to find a used or clearance P-60 for right around your price range.