Can I put Radeon HD 6570 which requires 22A on V12 if i have 18A ?

You should be fine, unless your CPU is some massive monstrosity

The 6570 should draw more than 6 amps on the 12v
Consider the 6670 that only draws 61W on max load

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If you're worried, a cheap yet decent 500W PSu will only set you back about $30 right now

http://pcpartpicker.com/part/corsair-power-supply-cx500
http://pcpartpicker.com/part/evga-power-supply-100b10500kr