Hello Maxaronnie, Welcome to the site.Check out this post:
http://forums.altimas.org/zerothread?id=437323I just suggested all the parts together for that guy. As for a head unit, I'd say go to BestBuy or something and just find a unit that you like. I looked at Crutchfild HU's but once I played with them I was like "No way.." User friendly-ness is a big part it. You gotta try it in person 1st.
Also be sure to get a HU with 3 RCA outs for (just in case) you want to upgrade in future or add on. Another thing to consider if you want to keep steering wheel controls to get a unit that has the adapter for it. Kenwoods do and that that brand I went with.
Are you going to be installing yourself or bringing it somewhere. If bringing then make sure to buy all the extra parts like, wiring harness, antenna adapter, mounting kit, your own RCA cables so you don't get burned by store prices. (3x~4x as much)
Also check out what I put together and the prices I got everything at here.
http://forums.altimas.org/zerothread?id=431683I did go with Infinity Kappas all around but you definitely need an amp to push them.
My major thing I don't like about them is I found I they had very little bass. Actually much less than the stock speakers so I have to go with now a 2nd amp and sub box.But in terms of clarify of highs they really are nice. Sometimes a little too bright where it makes you close your eyes or flinch.