Velero, try this website, put your data in and it will output a suggested PSU requirement, for your system:
http://extreme.outervision.com/psucalculatorlite.jsp
Also i recommend my PSU Guide in the PC Hardware/Software forum here:
http://www.subsim.com/radioroom/showthread.php?t=182356
Edit: A too low a wattage psu can cause all sorts of trouble for you, loss of data, fried hardware, the list goes on.