I upgraded the Operating System on my 4 year old iMac last night. We went from OS 9.2.2 to 10.3.6. (OS X Panther)
We probably will need to add some RAM. We have 256M, and it will run with that, but Rod told me that it will run much better with more.
I guess I should thank Microsoft. Internet Explorer for MAC has become increasingly buggy dealing with Java Script and Shockwave flash, both of which have become ubiquitous on the web. Problems with Internet Explorer were what finally motivated me to make the upgrade.
And to you Windows XP users who finally got an OS that approached the MAC, well, let me just say, you don't have squat.