Will DDR2-1066 memory work with an AMD Athlon 2x64, 6000, 3.0 ghz CPU ? The motherboard will accept it,but will the processor function with it or must I switch to DDR2-800 ?
The motherboard shouldn't even have a setting for DDR2-1066 with that CPU installed. The RAM setting in BIOS should be DDR2-800, and the CPU will run the RAM at about DDR2-750 (since the 3.00 GHz IMC doesn't divide evenly into the 400 MHz signal speed of DDR2-800.)
If you want to run the RAM at full DDR2-1066 speed, you need to overclock the HT bus by 33% to 266 MHz while leaving the RAM setting at DDR2-800. Remember to drop the HT multiplier to 3x since A64 X2s hate a HT bus >1000 MHz and also drop the CPU multiplier from 15x to something more appropriate like 11x-12x to keep the CPU at a reasonable speed (3.067-3.333 GHz).
The old K8 Athlon X2 processors only support a max of DDR2 800, anything higher get down clocked to DDR2 800. As MU_Engineer stated, you can than overclock in order to get the ram running near 1066 again.