Hey Myriad...this is my gut-take on your question...you have 4 options that will give you a palette (a wide variety of sounds) to work with....with some varying degrees of complexity to use, price to pay, and quality of sound...
softsynths that work as software in your computer and is manipulated by a midi-equiped keyboard (I have little knowledge about these, just what I have read on this board)...from my readings these seem to be relatively inexpensive ($100 - $300 ???) if you already have a proper computer. I haven't had good experieces with MIDI...others couldn't live without it.
Brand new, top-of-the-line, keyboard/synths that can run from $600 to $3000 and beyond. I don't have one of these but it makes sense that these are going to sound great.
Past top-of-the-line keyboard-synths that were produced in the last 10-12 years which can often be found for $300 to $600. I still have
my Roland XP-50 from around '94 and it still sound impressive to me (although the sequencer is notoriously un-user friendly)
The "home" keyboard with built-in speakers...usually a Yamaha or Casio...this is what I have been recommending to ones who don't know what they want or where to start:
the Yamaha PSR-273 is $157 in Musician's Friend. It has 480 voices, 32-note polyphony, and touch response. (It does not have a pitch-bend wheel, with some here feel is essential)...If you can check out a Yamaha model in a department store and see if you like the sounds...I have
a PSR-540 and I like its sounds.
you could also go to a music store and check out the "big boys" and compare...I guess a lot depends on the money you have to spend and how much time you want to devote to learning what is out there before you buy...Hope this helps you.