Originally Posted by Rhysc
Sorry I meant the cooler dissipates 250 watts
250w definitely too low. You could get away with 500w easily, even on an Overclock. I tend to leave some wiggle room for future upgrades in my builds though, and since 600w-650w tend not to be much more expensive, I would grab one of those. Just make sure to get yourself a PSU that is at least 80+Bronze Rated from a Trusted Brand (Like EVGA, Corsair, Cooler Master, Seasonic, Fractal Design, Be Quiet etc...) and not some off brand not 80+ rated crap, and at least 500w, and you will be fine!
The calculator that
posted a link to is reliable as well, so if you want to plug all your parts into that, that should give you the bare minimum you should go with. Don't be afraid to get above what you need though. Some new builders tend to think that getting a PSU bigger than what they need will pull more power, this is a misconception, the PSU will only pull as much power as your System Needs, so even if you got a 1000w PSU and your system only pulls 300w under full load, you are still only going to pull that 300w from the wall, plus a tiny bit more lost to waste heat (which would happen even if it was a 300w PSU). Also the efficiency curve tends to be best somewhere between 50% to 65% of the full capacity of your PSU. So if your build is expected to pull 300W at Full Load, its good to grab a PSU that is somewhere between 550w to 600w, because that means it would be the most efficient (less power lost to waste heat) when you were at full load.
Anyways, hope this crash course helps!