SAPPHIRE have just issued out an article which explains the characteristics of AA, HDR, and HDR+AA on their SAPPHIRE X1K Series of graphics cards. They also go to the effort of displaying side by side screenies showing certain features turned on and off so you can clearly see the effect they have.
What are AA, HDR and HDR+AA? Can you REALLY tell the difference?
The two main features that drive 3D image quality is High Dynamic Range and Full Screen Anti-aliasing.
High Dynamic Range is a lighting process that's been designed to emulate in-game or artificially generated lighting to closely mirror the changes we see in the real world. This technology can be found in an increasing number of modern computer games to make lighting effects with very high contrasts look as realistic as possible.