biznatch: the dual vs quad core debate has been around for a long time. Generally speaking, higher clocked 2's are better than lower clocked quads. With the exception of a few newer games. Some believe quads are more future proof..others say theres no such thing as future proof and point out the slow pace of adoption of software utilizing multi-cores.
I say split the difference and get an AMD tri-core that is somewhere in between obviously; more cores, higher clock speed.
But if you're stuck to Intel, the 8400 is generally known as better value than the 8500.