What is the make on model of your psu?
watts aint everything, you could have got a 500watt psu for about 30 quid, then you can get psu's that are 350watt-400watt that may cost more money, and those 350watt psu's or 400watt psu's could handle more power than those 500watt psu's, its the +12v rails you wanna be looking at, a psu with about 450-500watt and maybe 28A on the +12v rail is what you want, dual +12v rails are better tho.