I've had many questions about power supplies as well. It's confusing because there are alot of people that think more is better, and sometimes it is. But I have been running a socket 939 X2 system for 4 years with a 7800gt and an enermax 485W non sli psu. It's run flawless for me and I have 2 drives, 1 dvd burner, and a rather large swiftech water cooler. I just installed an xfx 9800gt 512mb and the minimum required wattage for it is 400W even though it draws only about 230W at load. Video card companies usually include a large margin for error in their estimated requirements. But I've seen some people try to tell me, of course not from this forum

, that I'd need at least 600W PSU to even run the 9800GT and something near 800W for sli. Well I'm still using my 485W and being a hardcore gamer on a budget it works out nicely, without upgrading the board, cpu, and ram in a drastic manner I see no reason to upgrade in my case.
I would imagine you could run an 8500gt on even less than 400W. It is always good to leave room for expansion but I'd say that most PSU's will outlast most of the parts under the hood if you upgrade at all and buy a reliable brand. Enermax has worked for me in the past 10 years that I've bought from them. But I've noticed that companies like Antec or Seasonic are also nice brands. My advice would be to get something that quality, look at the efficiency rating. Most psu's have 80-85% efficiency, get something like a 450W or 500W if you intend to upgrade at all down the road.
Hope that long drawn out speech answers some questions for you =)