A good set up?

You said you want to put a "nice system" in your vehicle--do you mean just subs?  Cuz if you're looking to improve the overall sound quality 4 subs is gonna be a little extreme without other add-ons to the system.  But if you're talking about just getting a whole bunch of bass, then it's obviously not out of the question.

Interesting idea runnning two 8s and two 10s/12s...I think it's unneccesary though.  First lemme say that all of this definitely depends on what kind of subs they are, what type of enclosure they're in, and a many other variables.  A pair of good 6x9s in the back, along with two 10s or two 12s, will give you great bass across the entire low-frequency spectrum.  I suppose it also depends on what kind of bass you prefer, and what type of music you listen to.  The thing is, 10s or 12s will go deeper than 8s, but 8s won't go a whole lot higher than 10s or 12s.  At least not to the point at which you could tell a huge difference, as a good pair of 6x9s will more than make up for any difference.

However if you're talking about extremely loud volume levels, throughout the entire low-frequency spectrum, perhaps even up to mid-bass level, I suppose 8s wouldn't be a horrible idea.  I really just think it's kinda overkill.  I wonder what everyone else thinks?

You asked what kind of amps you should get....well there's so many to pick from, first let's find out what your budget (if any) is.

When you say nice system are you looking for something that can be heard for blocks or something that has some good sound quality?  Also what do you plan for your mids and highs? What’s your budget? 

Sorry I'm still a little confused... are you just talking about just bass?  or are you talking about an entire system, including subs?