What you're saying usually happens with low quality PSUs, that don't really deliver the power they're rated for. (Anyway, I'm not claiming to know what PSU you used.) As for the OP's question, a higher wattage PSU could be used, and would give him more headroom if he'd like that, but his PSU would work well. http://www.hardwaresecrets.com/article/Antec-VP450-Power-Supply-Review/1487/7
As you can see in the review, it works just as well at 99.8% capacity.