Well, Core 2 Duo is old, I suggest you don't go for it. There are two different Core i5s. There are the Core i5 600 series and the Core i5 700 series. The Core i3 and Core i5 600 are dual cores with HyperThreading, and the main difference between the i3 and i5 is that it has Turbo Boost. The Core i5 750 is a quad core CPU.
If you are mainly using programs which only use one or two cores, or non intensive applications, I suggest you go for the Core i3 - the Core i5 600 series is overpriced. If you do game and a bit of multitasking, go for a Core i5 750, and if you are up to it, buy a Cooler Master Hyper 212 Plus for $34.99 and overclock it - you will see relatively large performance gains.