I have 1gb in my iMac G5 2.0ghz 17". Most of the time I have Safari, Mail, Photoshop CS, iTunes, iPulse, plus a few other apps and LOADS of haxies running. I also sometimes play UT2004, which runs great.
According to iPulse, I very rarely use more than 75% RAM, that is to say at least 25% is usually completely free - even when I have 10 or more CPU intensive apps open at once.
When I bought this iMac in May, I had planned to get another 1gb stick at a later date, but I really don't think my system is going to ever really benefit from it.
I would recommend you check out how much RAM you actually use first, to see if it could do with improving before you shell out any cash.