Car Part Question - What is this bit called?! Watch
ive always called it a bump strip, dunno if thats right or not
If you plan to replace one side it might be worth considering replacing both sides with silver or body-coloured mouldings (used on the Renaultsport Clios), although they might be harder to find than the normal black ones.
here it is again: http://www.cliosport.net will be able to tell you where the best place to get that part is, and how to fit it.
Just to appease the mods, before they moan:
http://www.cliosport.net/forum/forumdisplay.php?f=18 is the forum you need to ask in.