The Roland XP series IS dripping with potential, but it also hampers you with an operating system that can lead to pulled hair. Likewise with most Yamaha products; they seem to start over with a different operating paradigm for every new unit! Maddening!
I'm pretty biased in favor of Korg; the N and X series would fill the bill nicely and are much more user-friendly. I prefer their overall sound set. Going with a synth that has a disk drive is ALWAYS preferrable as well.
Do check out the Rolands, because their sound expansion options are massive and definitely in the top 3 quality-wise. If you've heard any of their sample CD-ROMs, you'll know what I mean. Just factor in the learning curve. If its too steep and you're not an oscillator-head, you can often find yourself pushing buttons and saying bad words instead of making music.