why do you need a mixer if you only have the mpc now and no other gear? Do you have the 8-out card installed or something? As you can see, most people here have a million samplers and modules hooked up to a mixer. That's why they need a mixer.
I would suggest getting some more stuff to hook up your mpc first. You already have a computer, so why not buy a zipdrive and some nice programs to make your sampling life easier (Wavelab, ReCycle, MPC-Editor, Cubase ).
And since you asked about a soundcard: Why not buy a soundcard with multiple inputs right away? That way there wouldn't really be any need for a mixer ever again. I don't know if you have a pc or mac, so i can't be to specific, but check these sites out for soundcards:
http://www.m-audio.com
http://www.motu.com/
http://www.terratech.com
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my own gear:
mpc-2000xl + zipdrive
Waldorf Pulse
Yamaha tx16w sampler
Crumar bit 01
2 vestax decks and a mixer
2 PC's with m-audio quattro soundcard
Mackie 1202 mixer