Most people underestimate the importance of a quality power source. A low quality power source can generate alot of waste heat, go bad taking your motherboard or cpu with it, provide underrated power levels, provide unclean power with load spikes, etc.
You should first calculate your power needs. Here is a good resource for you to do that:
Power supplies reach their peak efficiency at around 80% load. So get one with some breathing room, but don't get one that is over 2x what your needs are. Get one with enough power for your next generation video card.
Then look for the highest efficiency power with modular cables so you only plug in the cables you need. This will help with airflow, dust cleaning, maintenance, upgrading, sanity.