Running a 600HPS is best accomplished in a space at least 4x4 feet footprint and 8 feet high. Your closet size is appropriate for a 400HPS.
It's good that you have the height. You will need it to put your exhaust blower. Rememnber a light and its exhaust should be considered a single unit- buy them together to suit your space.
Tinfoil is crappy. Paint it white like the Pimp said.
Plan to keep your ballast up off the ground, preferably outside the closet.
Install a fire detector and keep an extinguisher handy.
Plan how you will get stale air out of the closet- if you rent, it may mean buying a spare closet door that you can cut a couple holes in, and building a baffle over the hole for the end of the duct and a flap over your passive intake slot at the bottom.