I have a similar system to what you say above. Core i7 with ATI 4870 x2. I am using a cooler master real power 850w PSU. It runs my PC perfectly in a Antec 1200 case with 3 hard drive and all fans connected.
Here is a useful link to a site where you can work out the power requirements for what ever set up you feel like having.http://www.extreme.outervision.com/psucalculatorlite.jsp