well you did try an ati card, which does not even support CUDA so i kind of ruled that out. So what are you using it for? I suggested a gt240 because there should be less bottleneck and it would be compatible.
using it in a htpc that does encoding. a fast gpu allows me to do gpu accelerated encoding. turns out that the cause of all the problems is a hardware conflict with amd 5xxx series gpu's and the older nforce4 north/south bridges.