The stated tdp for a hd7850 is 140w, but it normally only uses about 110w when gaming(if you don't overclock). If you do overclock(highly recommended to get the most from a 28nm gpu) you won't exceed 150w per card(unless you do some hardware modding, which is not recommended). Your i5 3570k is a 77w cpu(more like 100-125w if you overclock which is recommended to get the most bang for your buck out of that unlocked cpu). These parts will pull the vast majority of +12v power from your system(compared to other parts). Even when overclocked, those parts won't pull anywhere near those wattages during normal use(ie gaming). Only if you did something like run prime95 and furmark at the same time would you be close to max draw, and even then you wouldn't be close to those adjusted tdp numbers. The xfx 550w(built by seasonic) can supply more than its rated wattage(if pushed to extremes). Even with capacitor aging, your xfx 550w should power your i5 3570k(overclocked) and 2 hd7850s(overclocked) without issuefor years. 2 hd7850s are similar in power draw and performance(both stock and overclocked) to a hd7970(stock and overclocked). I've recommended xfx 550w for configs with a hd7970 and a more power hungry phenom II x4 965BE(overclocked to 4ghz+). As long as you aren't using a ton of HDDs or optical drives, you'll be fine.