I belive that its not worth upgrading this rig. You'll spend quite a bit of money on getting legacy pieces such as DDR2 memory and other hardware compatible with little performance gain. Just start a complete new rig and you'll be fine. What's the budget?
Well, if you don't plan on spending lots of money i'd certainly recomend AMD's FX solutions for the CPU. They have the best price/performance ratio and really deliver great budget gaming horsepower. If you are willing to spend more, go for an I5 3570K. The best solution for gaming today. As for GPU... well its hard to tell without a budget. For around $310 you can either an GTX 660ti or an HD 7950. Depending of the games you play one will perform better than the other. Thats up to you. For the Ram, get either OCable 1600 Mhz or sock 1866 Mhz. Mobo get any reliable brand with the most recent chipset and you'll be fine. PSU, at least 650W with 80 plus bronze certificate. I guess this is all the advice i can give you without a budget. When you chose ur budget, please tell, it will be way easier to assist you. :P
I would just get a cpu cooler for like $30-40 and overclock your cpu to 3 GHZ +. your hardware is old but still decent then you can test out see if you need more or not.