Most guides I've seen for calculating power requirements totally suck. I just downloaded the Overclockulator thing and it's not like it was made by complete idiots, but it's certainly not giving you the big picture.
You really can't make any sort of guide without having ridiculous genealizations. Overclockulator says a HDD uses 25W, a CDR uses 20W and RAM uses 12W. Things just aren't that simple. A hard drive can draw up to 35W when it's spinning up, but it'll draw a little less than 15W during heavy use. You also have to look at the rails. The important thing to pay attention to is usually devices using the 12V1 rail. A hard drive will pull power off the 5V and the 12V rails. They say RAM uses 12W, but that depends on what kind and how much RAM you have. Besides, RAM only uses the 5V rail. A CDR can draw 20W, but that's when you're actually using it. It's not like you're going to be spinning up all your hard drives and burning a CD at the same time, so adding them together is irrelevant.
300-350W are more than enough for almost everybody who isn't running some ridiculous SLI rig with dual 6800 Ultras, but the Watts in themselves don't tell you much of anything about a PSU. Amps are more important. There are PSUs out there that are rated at 400W but only have 10A on the 12V1 rail. Seeing as a hard drive can draw a little over 2A on 12V1, and a GeForce 6800 Ultra can draw 4.22W on 12V1, it's easy to push 12V1 too far.
A lot of times you'll be better off with a 350W PSU than with some 500W PSU that doesn't have the power in the right places.
