The problem of lightning is that in the real world lights have unlimited lightning, and the human eye has a perception of 14 dB (10^14:1), but the video card using a standard 32-bit integer buffer is only capable of reproducing 2.4 dB (255:1), because it uses only 8 bits to store each video component (R, G, B and alpha).
Radeon X1000 series uses a 64-bit floating point register for HDR, using 16-bit for each video component. It is really interesting to note that GeForce 7 series from nVidia uses a 128-bit floating point register for HDR.
On the other hand, Radeon X1000 series allows the use of HDR and multi sampling anti-aliasing (MSAA) at the same time.