This site calculates watts and 12 volt amps needed. Not sure if the numbers are accurate but the layout is nice and the amps for the 12 volt rail is the most important information needed. Take a look.

http://web.aanet.com.au/SnooP/psucalc.php

Interesting. We always recommend this one:
http://extreme.outervision.com/psucalculatorlite.jsp

One detail, though. This one calculates giving some margin. So you have to change the configuration to make it calculate as using 100% load (the default is 80%). Then you multiply the number you found by two. For example, if your computer is pulling 200 W at full load it is recommended a 400 W power supply.

The reason is efficiency, read it here:
http://www.hardwaresecrets.com/article/181/9

Cheers,
Gabriel