I'd buy a pair of these for the fronts at least:
http://www.audioque.com/AQ-TS65_p_144.html
I'm going to buy them for my car in the next day or so. And at $84.15/pair on sale right now, plus free shipping, makes it an insanely good deal.
However, if you listen to dubstep, you're missing out on a lot of music without a subwoofer.
I personally have a DC Audio Level 4 12" for the low end. Overkill? Probably. But I like it. Just bought the gf an audioque hdc2.5 10" sub for her vehicle. Now I just have to find an amp for her.