Use the CPU to do folding@home or seti@home.
Find a BOINC project that suits, seti isn't it - normally too much politics.
Malaria Control is one of the more worth-while projects (I have a friend that supports it), but there are a few good physics and bio-chemical projects if you want to stay away from the number theory ones. Something like 70 different ones to choose from, mostly CPU, some ATI, some Nvidia.