I have a 650 watt psu in my desktop, my pc is now completely dead?

You can install any kind of PSU to your PC, but first check your M/B and your CPU, they may be dead.

if you have a standard case then any standard PSU will fit.

If you have a micro or slimline case then these are often a custom size.

you can use any size output you like as long as it is sufficient for the hardware installed. What you really need to get is an 80+ rated one, far more reliable and stable than those which are not approved but as you would expect, sometimes more expensive. Buy a cheap one and you usually get what you pay for, so to get value for money have a look at the Corsair Gamer series and assuming budget permits, these are the PSU to get for excellent quality at a reasonable price for a quality built product that is stable under load.

when are you people going to learn how a computer really works??

it's AMPERAGE, not watts that determine a supply's power. there are websites that will tell you what psu you need by indicating the hardware you have. this includes sustained amperage across rails.

you better hope your psu didn't take anything else with it when it died..

Pretty well any kind of computer power supply will do if the wattage is high enough. What you are doing is correct. The question is could something else have been damaged as well? All you can do is try & hope.

now it could be dead if not does your PC have good air flow because you need to keep it cool or it will over heat is it a own made computer ?