You will certainly see improvements but not massive ones. The GUI and things like full screen video at higher desktop resolutions will improve but you still won't have Core Image support.
The best vid card a Cube can take to the best of my knowledge is a Geforce 6200 which would make it the only Core Image capable Cube compatible card. Having hardware support for CI in Leopard is essential because if you don't it uses the CPU to do all the CI rendering which while minimized in that mode is very CPU taxing. I have seen up to a 34% decrease in CPU performance without a Core Image capable GPU.