Wattage is not really the only number you should be looking at. It's the amps on the 12V rail that matter a lot (once you know how much total watts you need, that is). I have seen a rule of thumb that basically goes something like: You want about .08 of the total watts delivered in amps on the 12v. (And of course all the over current safety measures and such that come with quality units). This is because the biggest power users (CPU/GPU) and things like fans use that 12v for their power. So for example my unit at 620 watts delivers 48a on the 12v. Well, 620 * .08 = 49.6 That is pretty close and a fine unit. Again, do your own research just to be sure, I'm just relaying what I remember reading a while back, so it might not be 100% accurate.
Safety features matter too: A cheap unit that fails can take other stuff with it, a good unit that fails will shut itself down safely and not hurt other stuff.
With one card and a CPU most systems today are fine with a 500W psu being more than enough (provided the amperage delivered on the 12v rail is sufficient), but if you want a bit more detail you can plug your build into pcpartpicker and get an estimate on power requirements. Even that won't be completely accurate but give yourself some headroom and you should be fine. I use the Antec HCG 620
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371059
and I actually thinkt he OEM of that unit is Seasonic. While it's not their best work it is still a very fine PSU and should be more than enough overall power for most systems, with 48 amps on the (single) 12v rail. The exceptions are sli/crossfire setups you need more power, and here again the pcpartpicker build tool can help there. It's not perfect though, it estimates my 5820k as 140W (based on tdp) but I know from reading that chip can pull 300+ watts or more when overclocked and benchmark testing. Give yourself some headroom.
To get even more granular info on power use you kinda have to dig. Or use a meter that can measure power draw at the wall (I don't have one of those). And things like "tdp" aren't really indicative of max power draw. It's a heat thing, (thermal design power) and doesn't necessarily correlate to power use. Example my CPU is a 140w tdp intel chip, but overclocked in a stress test it can pull up to 350W through its socket. Or even 400 with Prime 95 which can be dangerous I have read. So I don't use Prime 95 with it (Haswell e). That's the kinda thing you have to dig around to find out, checking various tech review sites for the part in question.
This guy knows a lot more than me, check out this video, and others:
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Addendum:
I looked up that unit on newegg just to see if anyone reported frying issues.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817152042
Normally, newegg reviews are nothing to go by, you need professional reviews by tech people who actually tear stuff down and verify it works according to the specs listed. On retail sites people give 5 stars to anything if it lights up when they get it. That's fine, but it's not good enough to qualify as a real review in my opinion. That being said, I found this one funny.
This review is from: RAIDMAX Blackstone series RX-700AC 700W Continuous Power ATX12V V2.3 / EPS12V V2.91 80 PLUS BRONZE Certified Modular Active PFC Power Supply
Pros: -Lasted two years
-Didn't fry any components when it failed twice
-Raidmax replaced it speedily (see cons)
Cons: -Lasted two years only
-Something melted despite the temperature of all other components being fine, there was a melting plastic smell for days. Shipped it back to Raidmax, they replaced it. Though it seems like they actually sent me back the SAME unit, just with some extra thermal paste slapped on the component that melted. The new one failed after a few months, and had some white, melted gooey stuff inside it, which was probably thermal paste, and certainly wasn't MY thermal paste.
Other Thoughts: I was extremely lucky that nothing was fried, and I certainly won't be dealing with Raidmax again.
Manufacturer Response:
Dear Customer,
All of our PSU comes with 2 years warranty, and if you passed the 2 years warranty we usually don't give replacements.
We appreciate you choosing Raidmax products.
-Raidmax Support Team
Thanks Raidmax! lol
Granted, everyone has units that fail. But the good ones don't have as large a risk of taking other stuff with them when they do, and have longer warranties. The unit I linked above that I use has a 5 year warranty. Take from that what you will.