If your thinking of getting a good value for money Keyboard for use with your computer and on it's own I would have a look at the Casio WK3000 £230 or WK3500 (same but with a floppy drive)£300. I'm thinking of getting one of these, as the MIDI spec is great and it also sounds very good, much better than earlier Casinos.
It has a USB port so you can attach it directly to your computer and MIDI ports.

PP