Use about $100 of that to buy the materials to fiberglass new mounts so you can fit 6.5" speakers in the space. Then some high end coaxials or good comps in the front, like RE REs or Oz Vectors, and a moderate pair of coaxials in the back. Should leave about $100 for a nice 2 channel amp, and you can run the rear off the HU.
Little overbudget considering bad estimates on fiberglass and wire costs, but probably worth it.