Yes the MR-8 has "8 tracks," but one of its big weaknesses is the crappy built-in preamps and lack of mixing capability. I found I was really frustrated at the poor sound quality I got running an SM-58 directly into the MR-8. What made an enormous difference for me was adding a pre-amp (an M-audio DMP3), and then an equally enormous difference was adding a condenser mic, which I also use through the DMP-3. I haven't felt the need to buy a mixer yet, as I'm using a computer for mixing purposes, but I think you pretty much need one or the other if you're going to do anything serious with the MR-8. But if you get a mixer with decent pre-amps built in, then you potentially kill 2 birds with one stone!
The Yamaha MG series mixers are awesome for the money. You can get the MG 10/2 model for $90 on eBay plus shipping, or $99 at Guitar Center. The 12/4 is a great step up from that, and it'd be a LONG time before you'd outgrow that one, but then again it costs $199. You'll hear lots of different opinions about the Behringer mixers...personally I think the UB series is pretty well-constructed and has reasonably good pre-amps. I think it'd be hard to go wrong either way, with a Yamaha or Behringer UB mixer (just don't get one of the Behringer MX mixers)