Sorry to tell you, you won't see a significant change by upgrading. As you already know, both of those chips are old, and while there was a difference when they were young, that difference is tiny compared to what a new chip+mobo would do. Unless the athlon was free I wouldn't bother.
Places like frys, or even bundle deals at newegg can get you a really cheap mobo and a modern chip bundle for $50-70 if you look hard.