core2 e8500

I am using an old PC with a Core 2 Duo E8500 processor @ 3.16Ghz. ( which supports DDR 2 Memmory ) The motherboard is an ECS - G31T (Dual-channel DDR2 memory architecture - 2 x 240-pin DDR2 DIMM socket support up to 4 GB - Support DDR2 667/800 MHz Memory) The PC works fine with 1 x RAM stick...