Like I stated before you want a margin of overhead in your PC power supply I usually go for a hundred percent, but even fifty percent overhead is better than none.
You can do this one of two ways.
Measure your PC's power consumption after it is built bue before you buy your video card.
Make sure to have the CPU loaded with something to do.
(this is what I did for the last three computers)
You could research what similar configurations consume and go with that.
Both CPUs and Graphics cards have a TDP(Total Dissipated Power) spec in Watts. Its supposed to tell you how much heat it makes, but since all of it comes from electrical input power means that its also roughly how much power the component uses in the worst case scenario.
So to get a good idea of how much power you need you sum up the CPU and GPU rated TDP spec plus add in 20 or 30W or so for the other components like the motherboard and drives. The number should be somewhere between 200 and 400 watts in most gaming PCs.
Okay so you come to the conclusion your beast of a PC needs 350W to run and you don't give a crap about overrating the PSU, it was cheep, if it dies il buy another one so a 400W PSU will do right?... Nope, it most likely wont. Power supplies are limited to providing a certain amount of current. Supplies that have more than one output voltage like a PC power supply have a individual maximum current for each voltage rail. The way they get the 400W number they proudly stamp on the box is by adding up all the maximum output powers of all the rails. So in practice you can only get those 400W from it only with just the right balance of loading on the right rails. If all the other rails are under low load you might be able to squeeze a bit more current out of one heavily loaded rail but no where close to all 400W from just that single voltage rail.
Historically computers used to consume the most power on the +3V3 and +5V rails so PC supplies are designed to put quite a few of those Watts there. But todays computers use by far the most power from the +12V rail. So what happens is if you buy a PC power supply that has only like 10 or 20% headroom what will often happen is that the 12V rail will get overloaded while the lower voltage rails are under little load.
The simplest way to get around this problem is to simply overspec the PSU so much that it survives even if the computers pulls all the power from just one of the rails. This is why you see 600W or even larger power supplies in gaming PCs and why graphics cards manufacturers generally recommend using 500W or even 600W supplies with there cards. There is no way in hell you are getting a regular single CPU and single GPU PC to use anywhere close to 600W no matter how much you overclock it, but it makes sure none of the rails will overload.
If you still want to put a small wattage PSU in there then you can still add up the TDP of the CPU and GPU and check the specifications if it can provide that many watts on the 12V rail(Vcore for both comes from +12V rail). If you have at least 10% headroom there it should work. Idealy you want to over spec the PSU by 50% to make sure its running happy and at that point you might end up with that 600W PSU anyway.
So if you don't know what you are doing just stick to what wattage Nvidia or ATI tells you to use, otherwise do your homework to confirm the lower wattage PSU will work in your particular configuration.