Your CPU is going to bottleneck you to minimum frame rates so even if you spent $200 on a new GPU. You will still end up with poor low frate rates, they may just be stable.
(I have 7 pcs here. The one in my guest room uses a 2.4Ghz dual core with a HD 7750, previously with HD 4850 and HD 3870, the frame rate does not change at all showing the CPU is even holding back the HD 7750 when tested in League of Legends.).
As RDC85 said, "The ideal part is upgrade both..."
That said, your best bang for the buck without upgrading both is the GPU. The HD 7750 will certainly give new life to your aging PC. I upgraded one of my home PCs over Christmas (Q9650 & 9800GT+) to a GTX660ti and the performance increase was substantial. Of course, the Q9650 is nearly twice as fast as the E5200 in most regards so you may not see the same level of improvement.
BTW, is it really so hard to spell out complete sentences so the people trying to assist you don't have to decode your text?