Forget about HP, what you're interested in is PSI and CFM. You need to know how many PSI you need to run a given tool as well as how many CFM of air it needs. Most tools will give you this kind of info on the packaging (if not, call the manufacturer).
All that being said, buy as much compressor as you can afford, you will NEVER regret having too much, but you will ALWAYS regret having too little. Heck, I've got a 5HP 60gal compressor and I feel it's "limiting" my future since I'll need to buy another bigger one if I ever find a way to finance an Iron Kiss air hammer from John Larson.
-d