please help in selecting smps or psu

[shaad2811]

hi guys this is the first time i am assembling my pc, i have a core i7th generation processor with asus h87 pro motherboard. I was going for a cabinet with 400watt smps, most of them told me that my pc wont start at 400 watts i need atleast 500 watts. Now please suggest me if i should buy an extra 500 watt smps or a psu..whats the difference between them, which one would be fine, and recommend a good brand.