I already thought that site was already linked here, so I personally didn't bother possibly reposting it. But been using that site for about an year and it gives a decent estimation how well my PC can run a game, though you can't always rely on the written requirements on the backside of the computer game case.
Sometimes they might be way higher then said there (Supreme Commander is a good example here, anyone been able to run it yet on the 81x81km map with 7 AI + 1000 unit max and still get a smooth performance out of it?)