Your basic question is "How far should I stress my PSU?". The answer is : As little as possible. I never ask a power supply to run at over 50%. The benefits are too numerous to list here. The extra money you spend is the cheapest insurance policy you'll ever buy. My advice is to always buy the highest wattage QUALITY PSU you can afford. Plenty of people will tell you it's unnecessary and they are right. It's just the smart thing to do.