In terms of graphics cards, you should make your game be able to figure out at run time what power and features it has, and then enable/disable special effects based upon this. Most big games have an options screen where the player can set up their own graphics quality, so even if your default settings don''t work well on their computer, they can edit it themselves. For example, say you have made code to do real time reflections. You could detect whether the users computer supported cube textures, and if so, enable the reflections effect.

I''m not sure where exactly to draw the line, because it''s always moving, people are always upgrading etc. But when you come to make that decision, bear in mind that the average gamer is likely to have a slightly better graphics card than e.g. a business user, but maybe not so much RAM as your average programmer has bought to support all his IDEs etc.

In terms of API, I think it is fair for you to demand that the player has installed a specific API. It is benifical to the players to upgrade, because many other games may also use the same API. It takes alot more effort to support multiple APIs than it does to support multiple graphics cards. Some games support both DirectX and OpenGL, but unless your game is mega popular selling millions of copies, I wouldn''t bother going to that much trouble yet.

The minimum spec of the game I am working on now, is DirectX 8, 32MB RAM, 8MB video card, and Pentium 2 processor. But on video cards with more memory, higher detail textures are loaded. Some special effects can be enabled/disabled. There isn''t any pixel/vertex shader support planned yet for the newest graphics cards, but I will implement that if I can.