I would recommend a Corsair 550W. Using this: http://extreme.outervision.com/PSUEngine you can calculate your power consumption pretty accurately. Your PC's is approximately 388W at 90% load.
You always want headroom so go for 450W at the minimum. Better safe than sorry though so get a 550W if you can. Corsair is IMHO the best brand for psu, though there are many other great brands such as antec, etc.
The more watts the better but its of course more expensive. Get the most you are willing to spend! That link I indicated simply helps you select the cheapest option. I can tell you with confidence that a 650W would be more than enough; I am running multiple drives, 5 fans, an overclocked 955 and a 5970 on a Corsair 650W and I've had no problems at all.