Then you can try this page where you can select the components of your PC and that will give you a decent (but also generous) estimate of what power supply you need.
(Formerly) Antec Power Supply Calculator
New link: Power Supply Calculator
Also, pay attention to this note on the page:
The total PSU Wattage this tool recommends will give a general idea of the range of continuously available power (not peak power) at which you should be looking. But if you are planning to build a high end gaming system, total Amperage available on the +12V rails—and how that capacity is distributed—could be as or more important than total Watts of power.
So once you have established the likely power needs of your system, please make sure that any PSU you buy will provide sufficient Amps of current on the various rails for all of your devices, and that it will have the proven reliability, service and support you deserve.
For example, at the time of writing, some high-end graphic cards require 22A to 25A ONLY for them on a 12V rail.
(Amperage was the cause for the troubles Caveman experienced with his new system not long ago, as he posted on our forums.)