I would upgrade the GPU first, but instead of going all out on the GPU I would make sure you aren't going to be restricted by anything else. Do some research but with the age of everything I would recommend upgrading multiple things to ensure you are getting good value.

But to answer your question directly the GPU is most important followed closely by the CPU

Do you have any recommendations, Ive been looking at track racer mostly

okay thanks, I always have a habit of looking at the more expensive ones.