People overestimate component wattage & underestimate PSU wattage output.
This is a failsafe way of doing things but can lead to putting you over budget when money is tight.
Give me an hour or so mate - I'll rig a power meter upto my fx8320 build with 280x just do you can see how much power it actually uses.
these reading taken at the plug
stock fx6300
-------------------
idle load 120w
full load cpu 242w
turboduo 280x oc full load gpu 380w
whole system stressed 560w
fx6300 oc at 4ghz
---------------------------
idle load 120w
ful load gpu 380w
full load cpu 260w
whole system stressed 582w
take the stock one ,allow for 80% efficiency on the psu
works out to 448w power wide system draw of 448w including 3.3v /5v /12v lines
a good 500w would run this setup mate with a 280x - no headroom for overclocking at all though.
the 270x draws pretty much 120-140w less than the 280x so err on caution & call it 120w less & your system (fx 8320 with r9 270x ) is pretty much going to max out at 340w full system draw even under 100% load stress testing.
seeing as the seasonic has over 400w on the 12v rail alone then it will do the job absolutely fine mate.
I was just going to suggest as you have no interest in overclocking,that you could save some money & go with this board
http://www.flipkart.com/gigabyte-ga-970a-ds3p-motherboard/p/itmdt9h6w2wsfg2c?pid=MBDDT9H6W2WSFG2C&otracker=from-search&srno=t_5&query=am3&ref=c9f5a5a2-0097-4b68-83ae-6e1fab157df8
the m5a97 is better but prob not worth the extra to you personally if you're not going to push every last bit of performance out of it.
This would even allow you to throw in a half decent aftermarket cooler which will perform a lot better than the one amd supplies
http://www.flipkart.com/deepcool-gammaxx-200/p/itmd9hkatppgpy5y?pid=COLD9HKAARXXSRAH&icmpid=reco_pp_same_computeraccessory_cooler_2&ppid=COLDH22W6ZUWBC5D
as long as your case is big enough??