I was in the same situation as you, because I had a xp1800@2300 11.5x200. But then I purchased a 2500xp barton (locked) and OC'ed it to 3200xp (11x200=2200). For me windows seems to load a bit faster and is a little snappier. A big difference is when I encode DVDs, because it takes less time than before. I got a good deal on the 2500xp which is the reason I upgraded.
So if you want to upgrade it depends on what you are doing. If you are just going to be playing games and surfing the web, I say dont upgrade because there won't be a noticable difference. But if you plan on doing some video editing and the such there might be a slight increase but not enough to spend an extra $50-$70.