The quality of the PSU is every bit (and maybe more important) than the quantity of power it's rated to provide.
We like to see PSU's torture tested to 100% of its rated capacity by the better review sites like JonnyGuru, HardOCP, HardwareSecrets and a few others. If you get a top rated PSU from one of those sites it's less of an issue if someone looks to be nearing 80% capacity.
Since the OP's system is going to pull less than 200watts DC, maybe 260watts AC if his PSU is 70% efficient a good 400W PSU is certainly generous. Of course we're guessing since we don't know what that 12amp +12v PSU is. Personally, I'm thinking it might be some type of dual +12 PSU.