I think you will get enough benefit from an upgrade to get rid of the LGA 775 system.
Probably not a big boost from CUDA, but it seems to me since you want to game with the system as well, a GTX would be the best choice. With the given budget that might mean a GTX 570 today, but in two months I'm not sure. Hopefully HDD prices will drop a bit in the next month.
I'm also thinking the more cores you can get the better.