AJ's right.
Without knowing what kinda music you're into and what kinda sounds you're after it is
difficult to make a suggestion.

You see, there are lots of tweakable polyphonic midi synths out there. SOme come as keyboards, some come as modules. Some feature just one form of synthesis, some are packed with lots of different forms of synthesis and options. Once I know what kinda sounds you need and what your musical direction is - I'll be able to help.

-ED-