You didn't mention what OS you have on this machine, but I wanted to mention that there is a huge difference between the performance of Tiger 10.4 vs Leopard 10.5 on a machine of this speed. Tiger does not require the same resources as Leopard, so if you have Leopard installed, you may want to downgrade to Tiger, and you may get at least part of the speed increase you're looking for.
Besides that, unfortunately this is an old and relatively slow laptop that isn't getting any faster, so you may want to consider upgrading to a used MacBook in the $500 range, which will be a night and day difference in terms of performance.