Platinum if you can afford.. else gold is also beautiful.. I rock a bronze.. it doesn't matter much unless you are like Quad SLI/X-Fire or servers or something that's extreme high end to go for higher levels.. if the pc is solely for gaming, go for the bronze and save a lot of bucks.. that's just my two cents.. but be careful as your whole PC may make up or burn up due to this one thing where most people are negligent about coz this is the sole provider of quality food (energy) to your whole PC and enhance your gaming/work/extreme experience..
The thumb rule is to shave off 20% of what the manufacturer says.. for e.g. a 500 watt PSU, 80+ rated of a good brand (xfx, seasonic, rosewill etc..) will provide you with 400 watts of guaranteed power supply.. so calculate according to that and buy a PSU..
Another way is www.pcpartpicker.com.. that website states the maximum required power for your pc on the top right corner after you select your components. You can take something based on that as well..