I would say the graphics card is, if only because it does not support levels of DX that some modern games require; the slow CPU will multiply FPS by some fraction, but in this case the missing capability of the GPU will multiply it by zero.
Even a HD7750 would be a substantial improvement, and should run on a PSU that can power a 8600GT. A card any stronger than that would be too badly bottlenecked by your CPU to be worthwhile. If you want nVidia-based features like PhysX, the closest equivalent is the GTX650.