PC Games Hardware tested a bunch of games at minimum visual quality and determined what the minimum hardware requirements are. You can check out a bunch of screenshots over here.
PC Games Hardware has been checking for quite some how a game looks on its minimal details. It can be assumed that the minimal quality settings are also used to determine the minimal system requirements. In the following we want to present some examples of recent games: What is the minimal visual quality achievable in the game and what kind of hardware is required to run it. So in this case we take a different approach than usually and don't show the maximal possible graphics but what the developers intended for gamers with weak PCs.
Here's a screenshot from FIFA 10 at the lowest graphics quality settings. Looks pretty poor considering it still requires a 2.4GHz single-core processor and an ATI Radeon 9800 card.