High Dynamic Range (HDR) is a newer feature and only a few games utilize the technology. To comprehend what it is about, though, you first must understand the concept of dynamic range.
When the human eye sees the moon at night, it looks bright because there is nothing else around it which is brighter. If that same person were standing in the parking lot of their local mall, the moon would not appear as intense due to the bright lights in the lot. Dynamic range is therefore the ratio between the brightest and darkest parts of an image. In computers, the values are defined as units between zero and one, while in the real world, the range is between zero and an infinite number. For example, the sun is considered to be 10^8 cd/m^2 or 1,000,000,000 candelas per meter squared.
Here in Far Cry you can see the lighting differences.
HDR refers to using technologies that help computers capture the higher dynamic range that the eye can detect. With help from firms like Industrial Light and Magic (a Lucasflim Limited Company), the HDR standard OpenEXR has taken a foothold. ILM has used it in films like Men in Black II and Signs. At this point you might be asking yourself, what does this do for my games? The technology allows developers to recreate "real world" lighting scenes in the surreal space. Titles like Splinter Cell: Chaos Theory and Far Cry (Patch 1.3) have taken advantage of this technology.
As you can see from the shots, the lighting from the headlights is extremely bright. You cannot even see the blades of grass that are in between you and the jeep. You can also notice the aura or halo around the torch. Finally look at the sky, and you can see the illuminated clouds - just like they would be in real life. HDR brings the best "extreme" lighting out in games.
Okay so it starts with an "L" and not an H. We can only be so creative sometimes. :-)
Regardless, with its Composited Desktop Engine, the 7800 GTX is Longhorn ready. According to NVIDIA: "In addition to supporting Microsoft DirectX 9.0 Shader Model 3.0 (Microsoft DirectX9.0c), the GeForce 7800 GTX fully supports and is optimized for Longhorn's Windows Graphics Foundation (WGF) 1.0 and Longhorn Display Driver Model (LDDM)."
It looks like the first beta will hit in August. But with the 7800 GTX, there is no need to worry about being ready for Microsoft - You are prepared!