I have an IMAC and want to upgrade the OS

I am Extremly new to IMAC, or Mac , have been a PC guy for a long time, But i have figured out the imac pretty good, I have a new background, sound works fine, it is on the internet fine, only I am not sure I should upgrade the OS being the system does not have much memory, and has anyone upgraded the OS, and if so how is it,??

Juice,
:biohazard

Well, the more memory you have, the smoother things will run on the newer versions of the OS.
There is no reason why your iMac (any kind) wouldn't be able to run 10.4.
Just max out the memory and put as much RAM in as you can afford (at least 512 MB) and you should be okay.