I agree with skye23412. I have 2GB on my PowerMac, and use MenuMeters to show me a continuous measure of memory usage in my system menu bar. Even under heavy load (Photoshop, Safari, Thunderbird, Skype etc.) all loaded, I have never seen my Mac report full 2 GB of RAM usage. Right now, bumping to 3 GB is likely not worth the expense.
Now perhaps if you are doing heavy video editing, it might be worth it.
I would recommend getting the iStat Pro widget and use it to monitor your memory usage. If you find that you *are* maxing out your memory, then it might make sense. Hopefully, per the above though, you will see that 2 GB is suiting you just fine.