As always, it depends. For games, yes, the cost is probably justified.
I just upgraded from a K6-III 450 mhz to an Athlon XP 1700+. I don't notice any difference in everyday apps. The only differences I have seen have been in games and A/V encoding.
Makes me wonder why I upgraded, since I do those latter activities pretty rarely.
AFAIK, the differences tend to be minimal except in certain apps. Last thing to consider is that Durons only come in much lower clock speeds.