Is it worth buying Core 2 Quad Processor ?

Hello everyone, I've Intel Core 2 duo 2.66 with 800 FSB, and 2 Mb Cache, I'm intending to upgrade my processor to core 2 quad, 2.66 Or 3.0, uming that it will work great with games than core 2 duo 2.66. Is there a big difference in performance or i better keep using my 2.66 core 2 duo processor, beside that I have ATI radeon 3650 HD 512 Mb, i thought to buy ATI radeon 1 GB, what do you recommand to upgrade, the processor ? or my Games graphic card, your answers will b very appreciated. thanks in advance