Games are the hardest on your graphics card. Most applications (excluding rendering applications), older games and even some newer games are good. But when you play games as your primary focus for having a computer then good graphics aren't enough. Us gamers need GREAT graphics. I had a Matrox G400 and it will run better then my Radeon 9800Pro when it comes to rendering a 3D model in Pro/E, but with the Matrox half the maps for UT2003 are unplayable due to jumping.
The most powerful game engine to date is the CryEngine. Other games with incredible engines are Hitman Contract, Splinter Cell – Pandora's Box, UT2004 and one that is the oldest that is very strong is the C&C Generals (but this mostly due to the activity going on).
Right now the hardest hit on a graphic cards in games is fog. Fog in any game has the potential to maxing out a graphic card. Other elements that put strain on your graphic card is anything that is transparent, i.e. water, dust, clouds (only if you can see through them). These have the most texture driven components to any computer game.
I can't wait to see Doom III and Half-life II, these are going to eat the other engines…so they say. But wasn't Doom III and Half-life II supposed to be out on Halloween 2003!?! This release is like the Engerizer Bunny…the release date just keeps going and going and going.