For under $1000, ur probably looking at a DPMS. Standard barrel or M4 style Bushmasters and Rock River Arms can be had for less than $1000, but I usually find myself shooting from a bench, or at long ranges, so I prefer a full length gas system, and a heavy barrel.

The DPMS Lo-Pro Classic is a pretty good bargain ($800ish?). Get a new trigger (or a trigger spring kit), and a good optic and you'll have a real shooter.