Apple compatability

Right, here's the problem. I have an Apple ibook g4. I really like. I know some people think Apples are a waste of money, but the thing is my school had them so I'm used to them and also I was able to get one in a cheap deal. Its small and dinky and I feel happy working on it - at least until recently, where nothing is compatible. first off They only have me 256 mb memory, something apparently that Apple got a telling off for, the next models they have apparently a lot more memory( and I say apparently as I really have not researched it). So maybe it is possible to buy more memory at considerable cost, but does the memory have to be compatible too? The biggest problem is that my laptop runs on mac 10.3.9 panther, but I need to install 10.4 tiger. This will also cost. Thing is they are now selling version 10.5 and can't find tiger anywhere. The smartboard software is not compatible with my lap top until I can update. It's very very annoying and am finding more and more that I cannot download anything. I can't afford a new lap top so I'll have to find a way!! The trouble with apple is everything moves too fast with me to keep up with, or at least my bank balance. Still love them though.