Some of the hardware developers have created webpages over the past few years to try and help you work out the rough size of PSU your system needs. This includes some of the memory makes, the CPU makers and the PSU makers themselves.

But for me the best one is the Thermaltake Power Supply Calculator.

http://www.thermaltake.outervision.com/index.jsp

This is a very comprehensive calculator, giving you the ability to select all of the components you have in your PC, right down to selecting exactly what the graphics card is, how many IDE or SATA drives you have, and even the number of devices connected by USB.

This is quite an accurate tool. I worked my main systems power needs out just now and I was happy to find that my 600W PSU is easily big enough for the moment, with my current setup needing just 340W. And it will cope with the extra couple of SATA HDs and newer graphics card I currently have planned. All totally 430W at maximum load to all components at once (which rarely happens).