get a new rig ASAP. save up on money, then buy a completely new rig. i can recommend a new GPU now, but then it will be bottlenecked. if you upgrade the rig piece by piece, by the time you are done, the GPU is horribly outdated
TheBigTroll: Socket 775 systems can still be very capable. May not be current generation but certainly still usable.
I would recommend a Radeon 7770 as it will pair well with the strength of the CPU.