I picked up an M-Audio Keystation 88es for $100 a couple years back that suited me well until it was stolen at a gig. For the price, you can't get better: the keys were decent (semi-weighted, so feels good to play but not like lifting a car when you need to move it), real basic controls and features, easily expandable, and hey, 88 keys. It's single channel, as far as I know, but unless you're playing keys live and need to switch between patches in an instant, you really only need one channel. After getting jacked, I went for the pro version and managed to talk the seller down to $150 (pretty good, considering I see them going anywhere between $200-350 used). It has all the MIDI functionality I could ever ask for, and then some. Plus, it feels nice.
Both of those controllers are out of production now, so it's used market, but you can probably find something comparable from M-Audio, Alesis, or Korg. You could also look for a synth that has its own sounds, but I think it is more fulfilling in the long run to get a MIDI keyboard and go the software route. If you have a Mac, you have Garageband, which has its own MIDI instruments (Quicktime also has soft instruments), and I'm not sure what there is for Windows, but it looks like you can easily find free MIDI instruments. http://mustech.net/2007/06/midipiano...ynth-and-more/
I personally use a hardware synth, as I don't like having to have my laptop around in order to play my keyboard. There are numerous cheap hardware synthesizers that have decent bread and butter sounds (strings, horns, organs, vox, piano, square/sawtooth/triangle/sine wave, maybe some leads).