Although it's a bit late, I found two websites that allow you to calculate the PSU you'd need for your system:
1) <a href = "http://www.extreme.outervision.com/psucalculatorlite.jsp">Outervision psu calculator</a> (seems more up-to-date)
2) <a href = "http://web.aanet.com.au/SnooP/psucalcnew.php">Another psu calculator</a> (more details about the 12V rails).
I entered my (potential) future computer into the calculators, and got a recommended PSU of 340/350 Watts. The computer has a dual-core Intel chip, 4Gb of DDR2 and a Nvidia 8800GT videocard.