I agree with SAAIELLO. If you have $700 to spend, you might as well get a new computer. Upgrades would not give the best performance benefit. I would go with an Athlon IIx3 though. Higher clockspeeds and most games currently do not use more than 3 CPU cores.
I would suggest upgrades regardless as an option for you but I cannot, since you did not give us information on your motherboard, PSU, case, video card or model of pentium dual core or the type of ram. For example, if you had DDR2 ram, you could certainly reuse it in a new build saving about $100.