Whoah....
You have a wierd combo.
Ditch the 303 and the RMX1.
Totally....you don't need those toys. I know a lot of people on here give props to the RMX1, but fuck that dance market bullshit.
I would use the MPC as a Sequencer/Drum Machine (that sounds pretty ... empty, I mean...obvious, as that's what it's built for, but...)
I would use the external midi on the electribe to drive your Virus and (is that other Yamaha thing a tone generator? I think it is?) whatever to sample into the MPC to save polyphony. That seems wierd, but the reason that *I* do this sort of thing is to make sure that If I sample a sound, I can be sure that it has X velocity and Y gate time.
If I were you, my next purchase would be a rack sampler that is more synth capable. Once you get rid of S&S boxes, you start to get your own entirely unique sound because you literally make your own sounds.
I dunno...that's just my take.
I use an MPC and a JP8080 (which I'm trying to rid of). The MPC is of course my drum machine/sequencer and I ripped some necessary samples from my XP before I sold it (pianos, organs, etc.). Every synth noise I use is either sampled from my 8080, produced by my 8080 (directly) or ripped from the internet.
GO HARDWARE! I have camaraderie with us oldschoolers who still hardware sequence.
Oh, and by the way, that new 1.07 Update for the MPC is total crap. It crashes left and right. Stick with 1.06 until the next update. (If you try 1.07 and it doesn't crash when you do complex timing changes (changing global swing% or the velocity% for a whole track) let me know, maybe it's just my memory that's shoddy.