New to mac, help me please!

+1 in retrospect 10.3 would be better, i noticed my mac slow up a bit when i went to 10.4 but it was pretty much out of use by then so wasn't too much of an issue.

maybe bump your RAM up a bit as it's pretty cheap to do so. Crucial are pretty good for it
http://www.crucial.com/uk

Definitely upgrade the RAM if possible my 3.5 year old powerbook G4 had a second lease of life after I gave it some more memory, feels faster than my new work laptop running windows!