You can - from memory, it's something like <ignore_cuda_dev>X</ignore_cuda_dev>. But don't quote me on that. Check this FAQ item for info on the cc_config.xml file (there is a link there for all the options you can configure).
But, if the 9500GT is the weakest GPU in the system, and at least 1 other GPU is an Nvidia, the 9500GT will not be made to crunch by BOINC by default.