I've spent the last couple weeks shopping around for ARs, and I've come to the conclusion that you really can't go wrong with any of the major brands (Bushmaster, Armalite, Colt, Rock River, Stag, DPMS). You can save a significant amount of money (~$150 or more) by buying the upper and lower receivers separately instead of buying a complete gun.
I would look on the equipment exchange on AR15.com. The dealers there seem to have the best prices on new guns and parts and there's always good quality used stuff too.