I had 384MB for a long time before I upgraded to 512MB and felt a good boost in speed. That was with 10.2.8.

I had the last version of Firefox 2 on it (2.0.0.20 IIRC) and still have the archive for it if you need it. YouTube won't run, as it requires Flash Player 10, which requires Firefox 3, which requires 10.4.

640MB runs fine on my 10.4.0 iBook. (It's the max anyways)