When building a new PC, I sometimes struggle to determine what capacity (watts) power supply to use. Is there some sort of online calculator that exists that allows you to choose your various components and will calculate the power consumption?
Here you are!
eXtreme Power Supply Calculator Lite v2.5
But it does say it's for non-commercial use only.
I'm no electricity expert, but don't you just sum up the watts of each component?
You can also use the fabulous Kill-A-Watt.
Always go for an A brand powersupply. I am partial to Enermax myself, but there are many good makes out there now. I tend to overspec my powersupplies by a factor 2. A brand PSU's can deliver the power as stated on the label, but only tend to keep their fans quiet until around 30% of their peak load.
Personally, I wouldn't bother calculating it. I would look at the Motherboard/CPU power requirements, look at the recommended PSU wattage for the video card I was going to run, and almost double it.
High-quality 1000W PSU's are cheap now, so there is no real reason to skimp out.
The only case I could see myself actually calculating it would be if I was running some sort of multi-cpu/multi-gpu setup, or a home-grown raid array with 30 hard drives.