Recommendations for a beginner...

I've been playing guitar for a while now and starting to get serious about composing music, not just playing covers all the time. My band's sound is moving in a new direction (from Black Keys, My Morning Jacket, Shins, etc. to more of an Architecture In Helsinki, Flaming Lips, Of Montreal sound). In short, I really don't know much about keys and synth, but I'd like to try to learn. Can anybody recommend a beginner's synth for less that $1000, hopefully closer to $500? I know from looking around a bit that that probably won't get me anything amazing, but I'd like to get a keyboard with some solid basic sounds. Effects aren't too important... I own many already. A full size or close to full size keyboard would be nice to learn on, too. Sorry for my obvious lack of keys/synth knowledge, I'm as green as it gets.

Roland XP-30 with perhaps a few expansion cards. Lightweight, compact, decent feeling on the keys, and a boatload of useful presets.

Thanks, that's pretty much what I'm looking for. If only it came with the SR-JV80-04 and SR-JV80-08 cards... I guess it's priced well enough to expand if I feel the need. Is there much of a used market for keys like for guitars, or do people generally stay away from used? Maybe I could get the expansion cards used?

You can definitely get the expansions used - it's how I got the 04 and 08 . There is also a lively used market for keys. You could check the classifieds here or at vintagesynth.org.

People don't stay away from used; vintage synths command insane prices. Hard to sell used units are usually Yamaha PSR-keyboards or newer Casio arrangers, since they depreciate pretty quickly in value.