Billet heads make a huge difference in performance especially on the 5.0 engines. I'm running ProComp and am very pleased. They aren't too expensive and they aren't the cheapest either. I'm running the PC3038 series which has the 210cc intake port and 70cc exhaust. Intake valves are 2.02" and the exhause valves are 1.60". There are several options as far as heads go... To save money, some Ford guys go with the GT40 heads which also work nicely.
As for a cam, that's up to you and what you want the cam to do. Ford Motorsports has a great catalog with cams and are a good option to go with. You'll be running roller rockers so you can opt for a higher lift cam. I'm running the Motorsports cam. I've had good luck with Crane cams and Edelbrock makes a good one as well.