Aim for 600, but if there's a minor price to get 700 or 800, do it. Most PSUs are crazy efficient now-a-days and paying and extra $20-$40 for peace-of-mind can be worth it. But 600 should be overkill for your current setup and fairly future proof.
Make sure you review it first and get name-brand. Lots of PSU reviews from jonny guru/etc