I feel that most people are not just looking for the cheapest PSU with the least wattage to power their system. If they are, they would say so specifically and may need a bit more education on the following:
IMO: The choice should include three factors: Clean Stable Power, Efficiency, and How loud the unit will run.
When you run a PSU at peak performance, it is going to be less efficient, run hotter (although not necessarily dangerously hot for that unit), and therefore be louder due to the fact that most Quality PSUs will have self adjusting fan speed.
I would vote for something like a Corsair AX750 or AX850 if you want a little more headroom. Even for a single card. You can have an AX750 for around $150 and in the grand scheme of any quality build, that is far from a big chunk of your budget. Skimp on something else that is much easier to replace if you must or just wait an extra week or two and save a few more pennies.
I don't think we are doing anyone a disservice recommending a quality PSU in that range even for a single card build with no OC. These units will last and give headroom for just about anything in the future.
Really, what do you gain by putting a 500w psu in instead? Save 50 bucks, please. Replace the unit once and your're already loosing money.
For the Record: I am biased because I own one of each of the units I speak of. The AX750 is in my wife's rig Q6700 and GTX 295.
That said, informing people as to why a certain unit is recommended would also be kosher. Give them the choice of the minimum as well. I ran that Q6700 and GTX 295 on a cheap Apevia 650 for years with no issues.Edited by Brittain - 6/15/11 at 9:46am