Go with a high end Core2Duo such as the E8500 or E8600, quad core is useless for Dolphin unless you plan on using it for something else other than Dolphin. Current AMD cpu's are also slightly underpowered compared to Intel's latest offerings, even a Core2Duo E8400 - E8600 will blow anything out of the water AMD currently has to offer for dual core applications, and Dolphin is a dual core application.
The only reason a quad core might be beneficial for Dolphin is because Dolphin can have two cores all to itself without system processes or services hogging the cpu.
The only reason a quad core might be beneficial for Dolphin is because Dolphin can have two cores all to itself without system processes or services hogging the cpu.