Frankly, I don't think there are any appreciable speed differences between 10.2, 10.3, or 10.4. If anything, things have gotten faster with newer versions. (Benchmarks show this!)
Regardless, I run 10.4 on a 400MHz G3 and it runs just as well as 10.2 or 10.3. But you'll definitely need to bump the RAM all the way to the maximum.
tooki