I find pretty much all PowerBooks lag a little - even my 1.67 with the 128mb 9700 struggled a wee bit. I think it's probably more down to the CPU than the GPU. If you download Menumeters you'll be able to see the CPU rocket up while navigating around the dock.
Haha, I downloaded it, and yes, it turns out that I just had too much running :/ the dock just lags in general especially on 3D, oh well, I guess I'll just have to wait for my RAM to come.