Re: Advice on swapping Intel e6420 CPU for an AMD or Intel 64-bit processor
All of the Intel Core2 series should have EM64T. (Intel's 64bit instruction set)
Though, as said, if you still wish to upgrade, it will have to be an LGA775 Intel processor and match the Front Side Bus spec of your motherboard, which I would assume is 1066MHz. And the board has to feature a chipset that can support Quad cores if you plan to upgrade to one of those.
RD-PC -//- i7 950 // Evga x58 SLI // 3x2 OCZ PC3-12800 // 2x Evga GTX470 // Corsair HX850 // SilverStone TJ07