It's a good question.
You can't go far wrong with "Recommended", but you will be aware that what one game says as recommended may not apply to another game; meaning, all games are different. So if you are going to build/buy a computer with 'recommend system requirements' for one game, don't expect all games to be happy with the setup.
There's no way round that, and in fact the system you build/buy is likely to be out of date within a year or so of getting it. That's not to put you off, but hardware developers are always updating and "increasing the power of" their products, and it is nigh on impossible to keep up.
It's not just graphic cards, but dual core, quad core, I thought I read somewhere where 8 core is on the horizon, but don't quote me on that. And of course, RAM is so important nowadays.
Why do people in forums ask if their rigs can play the games? One reason I see, and it's not very flattering, they either don't read the specs themselves, or don't research it in search engines.
Good luck with you build or purchase.
I'm new to the "building/buying" a new pc for gaming. I'm at the point in life were all I do with a pc is either surf the web or play games. I see games have "recommended" system requirements... yet, when I look on forums some people have better graphics cards/CPUs and ask if it can play a certain game.