Pioneer do a set of straight swap component speakers specially for the Clio, that's the 13cm ones in the doors and the two little tweeters in the dash. A few on here have done that I think. They should be an improvement on the standard set up and would be the easiest if you wanted to upgrade the tweeters whilst your at it.
re. getting co-axial ones, you won't notice that big a difference between sets if your not amping them. Might be wrong though.