My Powerbook had a very similar issue, it got very very slow. I did several things, and they really sped it back up to speed. My test was always the fps on the iTunes visualizer because I could pull that up easily. I went from approx. 15 fps, to 60 fps (average) after this....
I used MacJanitor, then I used a program called Yasu (much better program). It does the same thing as MacJanitor, but it also does many other things. For example, it clears the logs that you specify.
Repair disk permissions through the disk utility. (Yasu does this for you, but I do it a second time. Why not?)
Reset Pram and Vram. You do this by holding certain keys at startup. You'll have to look them up as I don't remember them. (or someone might post it here...)
Doing those things above are good maintenance for your Mac anyways. Also, check in the activity monitor and look at how your ram is being used. See if there are any big activities that are eating up lots of CPU or Ram.