I have a 15 inch Powerbook G4 1.67 GHz, which I bought almost a year ago. When I first got it, it was a pretty speedy machine, after a few months it got slow, very slow. I have 1 gig of ram (same as back then), and it still felt slow. I realized that my widgets took up a good amount of system resources sometimes and I beleive I read somewhere that having too much mail (...in Mail) can slow it down too (Not sure if it's a myth or not). I had pretty much the same amount of programs running on it back then as I did today. But today it runs as fast as when I purchased it.
I simply rebooted the whole thing, started from scratch, and obviously it was fast since it only had the basics installed. I began adding programs one by one slowly and seeing any changes in speed or amount of resources taken up. Eventually I installed all my programs I had back then and uninstalled a few others (AppZapper is great at program removal) and now I had it at optimum configuration. At that point I decided to make a full copy of my HD onto my FW800 external HD, so in case anything begins slowing down for me again, I have my optimal config. saved on it so I can just put it back to the way it was.
I only notice it slow down when I use multiple programs at the same time, specially the types that take up a lot of resources, so I might upgrade to 2 gigs and see if I get a huge difference. Since i'm broke for now, however, this upgrade seems a bit far away.