How many watts should my power supply have?

as reasonably priced as PSUs have been recently, you might want to future proof yourself to some extent - if you upgrade that graphics card to a 1070 or 1080, that's going to want 150 to 250 watts by itself.

Just because a psu is rated 550 or 650 watts, doesn't mean it's going to consume that unless the system is consuming that much.

here's a Corsair CX550M 80 Bronze Plus for $35 after $20 rebate
https://www.newegg.com/Product/Product.aspx?Item=N82E16817139147&cm_re=corsair_550m-_-17-139-147-_-Product

just a suggestion
 

Karadjgne

Titan
Ambassador
Good idea, just don't go too far overboard, plan psu wattage @ double what you'll likely be using, that'll keep the wattage at @50% load which is the most efficient range, cheapest on electric bill, best performance from the psu and relatively best thermal performance. Slapping in a good 850w psu now because you might run 2x decent sli cards would be a waste of money all around.