Well, you would be surprised how quickly 512mb of RAM gets eaten up, especially if you are running other programs--even in the background. Firefox, especially, is a notorious memory leaker--and the FF compatible with 10.4 even more so. I would suspect that is your problem.
How to solve it? Depends on your machine. Memory is very cheap. IF it is a Power Mac 3,4 with dual 1.42 GHz, you can upgrade to 2 GB which will help a lot. You can also upgrade to 10.5.8 which will open your options more update programs.
--J.D.