All that matters is the 12v RAIL on your PSU.
Your Answer:
Your going to want a psu that provides at least 35-40 amps on the 12v rail, one like the Antec HCG 620 will be sufficient.
You really can't go to big, the psu converts ac to dc, and dc will only pull as many amps as the load requires. you can power something like a fan thats 12v 1a with a psu thats 12v 100a, it will only ever use 1amp because that is all the resistance the power has to overcome to complete the circuit.
Get a DECENT rated psu that has a 12v rail of over 35 amps.
if you have a 180w psu by the way, most likely your board wont be able to handle that GPU of that power, you will prob have to get a new motherboard and cpu.
You dont want to bottleneck your pc