It's the most important part of the PC. It regulates the power, protects the components from spikes and surges and powers everthing.
The size you pick depends on what components your PC consists of.
Pay less attention to the number (800w) and more to the amps on the +12v rail. All 800w psu's are not 800w. A cheapo or generic 800w psu may only put out 500w. A quality 800w psu will be capable of 800w+.
It's very important, probably the most important part in your computer. No computer can operate with badly working power supply. Moreover, it's not wattage that's important. It's the quality. To know which power supplies are of quality, you may check this chart:
How important is the psu and should that be a worry lets say a 460 as opposed to an 800?
Thanks for your answers and interest I got a decent not best but I think decent setup to game with on a custom HP tower for under 700 bucks shipped that comes with:
AMD FX-6120 6 core 3.6 GHZ 6MB L2/8MB L3 cache
Radeon 2GB HD 7570
and choose the 460w psu out of three choices:300w-460w-600w and went middle of the road, not knowing what a set up like this would need...HP wanted another 100 bucks to go from 460 to 600 but only 30 to go from 300 to 460 so being on a budget I took the middle. Should this be ok to run what I have showing? I don't plan on maxing games out just Medium or High is fine with me.
Thanks that answers a lot of questions but to be honest still scratching my head over these things as there is so much info and when trying to apply it there seems to be no 'pat' answers. Other than big and better is good but is it necessary? Like a car: do I need 800hp or will 300 do...my psu is a E131881 and looking at the UL listing the 3 12v lines are listed at 18.5 amps so what I plan to do is install an Radeon HD 6870 as the mobo has only one slot and no crossfire capability...grrrr. From what I understand about those GPU's they aren't as hoggy on amps and run cooler and with less juice.