Not to make the unpopular recommendation or anything... But I have had decent success with Win2k Pro on my Micron Transport XPe... Intel 150MHz MMX, 80MB RAM. Win98SE will also work well.
Any version of Linux would also perform decently. I wouldn't recommend DOS, since that's way more machine than DOS was meant for. QNX may work, if it likes your video chip. Likewise Amithlon would work, since a harddrive install is centered around QNX (that is, IF you can get your hands on it).
And last, but certainly not least, AROS would be fun to play with on it... Tho harddrive install is not yet available.
Hell, if you're feeling adventurous, you can always install Win98SE with a minimum install... Then use WinUAE in place of explorer... Change the SHELL= line in system.ini to WinUAE.exe :-D