Listen to dnj, he is a pro - his advice is solid.

I played both PSR550 and WK3700. I played for 16 years (in church) at least once a week, not like the "real" pros but I still log quite a number of hours and I was impressed by the Casio WK3700 and WK3200(cheaper but almost the same in function). If you hook up to the amplifier/your stereo system it will sound more than acceptable even to a lot of pros.

Another good advice is to buy it at places like Guitar Center that has 30 days moneyback guarantee just in case you don't like it. GC also will match any internet price like music123.com for example including the free accessories.