Also, consider that investing in a good PSU with a little more wattage means that your not sitting at 66% load all the time. There is also no room for overclocking your cpu, or adding another gpu if you wanted to. Also, keeping that kind of load on your psu can make it wear out faster so I've heard.
So much misinformation and BS being spouted out here.
OP. A Corsair CX500V2 is good quality (not the best but not shabby either) power supply that can pull and export its rated wattage with no problems at all. Your system will not break 300-350w under normal load and will only reach 400watts if you're extremely stressing your system with stress tests etc.
In Short. The CX500V2 will be sufficient for your CPU and GPU with overclocks.
+1 to all that Lilo said except you're power draw won't hit 400W with that setup including OCs to CPU/GPU. Take a look at this review of a system with a Factory OCd MSI Twin Frozr HD7870: http://www.guru3d.com/article/msi-radeon-hd-7870-twinfr... They hit 308W at full load with a highly OCd, power-hungry i7 965. You won't hit those power draw levels and will have significantly lower power draws when not gaming. The Corsair CX500V2 is a decent PSU and more than sufficient for your planned build and OCs.