GPU's are a lot focussed on what they do. As such the GPU's are a lot better at what they do.
Converting them to work on stuff other than graphics was fairly hard but now is a bit easier. Because they are so focussed on what they do, they give more omph to the crunching. One X1900 is supposed to be worth 4 CPU's once it's all sorted out.
It completes units in 4x less time.
When F@H finally sort it all out, you'll be able to run CPU alongside the GPU client to make use of it all(currently it's recommended to run only the GPU client on a single core machine)