While I totally agree that increasing RAM is absolutely essential, and the most effective way to speed up your mac (or PC), I have noticed myself that Leopard has slowed down noticeably after the last few updates.
I don't know what Apple are doing, but the last EFI update especially has increased my login times on my MBP - which made me resort to switching off my 3D dock, reducing my login items to just the iTunes Helper, and clearing my desktop completely just to get an extra second or two of speed. BTW, the things I just mentioned are also tried-and-tested ways to speed up your boot times.