A very good FAQ on CoreImage has been posted on MacRumors’ forum. One of the big questions is what happens when using a non-supported GPU. The answer:

If your graphics card is not supported by CoreImage then Apple provided a fallback option. You’ll still be able to use CoreImage but your CPU will do the work instead of the graphics card. This will obviously be slower than the GPU in most cases however it’s still very functional with a few catches. One of those catches is that you won’t have all the pretty effects and eyecandy you would if you had a supported graphics card.