apetaster

I am a Benjamin Moore guy, but any name brand drywall primer will suffice for right over bare sheetrock (spend the $$$ on a good quality topcoat.)

For your situation, however the sheetrock has already been effectively primed by the existing topcoat, so a primer is not generally needed (but a tinted primer to get you in the right direction before topcoating could save money, as primers are almost always less expensive than all but the cheapest topcoats. If a thin coat of sprayed paint is on there now and you are going for a dark color over thirsty walls, plan on three coats (one primer and two topcoats) for guaranteed consistency of color and no white patches.