Sean,
Soft synths are plug in modules that you can use with your sequencing program controlled by midi and then patched to the midi track you've layed down. There are tons of soft synths to choose from. The other thing you could consider is buying an arranger board and someone else's sound module for the synth sounds you want.

------------------
jam on,
Terry
http://imjazzed.homestead.com/Index.html