i have one from Radio Shack, witch I believe is a rebranded casio ctk691 -- and I like it a lot.
Of the "budget" keyboards I tested, it sounded the richest to me.
I compared to several low-end yamahas and one or two other low-end keyboards <$150.
Slightly an unfair comparison, because it was on sale at RS for $100 off (iirc) ($250-100=150... maybe $225-100=125)
This was a few years ago, so I'm sure it has been replaced by something with more features for around the same money.
I'm definately low-end as far as know-how/musical-talent... so a real musician may say I'm full of cr@p. All I really know is that it makes more pleasant noise than a similarly priced yamaha.
tips: Make sure you get one with spec "General MIDI" (your question suggests you know at least that much) -- and go somewhere where you can hear them. Best Buy has some on display... where you could hear casio and yamaha and maybe one other brand.
See if you've got a "Guitar Center" near you, check them out... they've got nicer stuff, all on display.
AMAZON $159 on sale, shipped. "only 4 left"