The PSU of a computer takes the electricity from the wall outlet and converts it to DC and it lowers the voltage to the appropriate values for your various components. The PSU itself doesn't affect your PC's performance. That is unless you have a faulty PSU or an inadequate one. If you get too weak of a PSU then your PC might appear to work fine but when you start putting a load on it, like gaming, stress test, bench marking it may cause your PC to freeze, reboot, or shut off. Its the same thing with a faulty one. Although a faulty PSU can eventually stop working completely, so your PC won't turn on at all.
As far as the wattage goes, it depends on your other hardware. You don't want to get a PSU with too little watts, but a PSU with too many watts isn't ideal either.
You don't want to cheap out on your PSU. You can find some pretty cheap ones, but you get what you pay for. The PSU does a lot for your PC, so if you cheap out on it, a bad one can potentially damage other components. Go for something like a Corsair or EVGA brand PSU.