how fast is that PIII?
if it is faster than 600mhz gnome, lxde and even kde4 could fit very well, i tested kde3 in a old ciryx c3 at 700 mhz and kde3 ran very well in livecd
the ram is the limit here, the board could support only up to 512 of ram and that could be the limit
when testing old pcs the big problem you could have is video card