It should not *require* DX 8.2, but the installer will probably try to force it unless you remove/replace the DX files in the directory that's included.
There were people transfering around their PC Gamer copies, and the orginal guys cut the DX Files out of the demo and replaced them with similarly named blank, 0byte files. In that situation, the game will install and give some errors on 2k and XP machines, but does require one of the DirectX EXE's to install on 98 machines.
To bypass them, extract the demo installer, and locate the DirectX diretory. Replace the files in it with 0 byte files with the same name, and you won't overwrite your directx.
For some officially un-offical information from Relic about DX8.2, see this thread:
http://forums.relicnews.com/showthread.php?s=&threadid=7332Note that EZ Pad is NOT used in this demo.