I've seen HP's gaming cases, you'll have enough room and it will all be compatible. I have a CoolMaster GX650w PSU and it performs like a champ, nice and quiet. As long as you buy from a company with a good return policy, that's well known, and that has <4 reviews, you should be fine.
NOT TRUE. Coolermaster is known for making HORRIBLE PSUs and the extreme power plus series is THE WORST. Do NOT buy one. Most can't even deliver their rated wattage, if they can they run out of spec, and they are prone to burning out. Return policy means NOTHING when your PSU takes the entire PC out with it in a blaze of glory