I have the opportunity to buy Corsair XMS2 4GB (2x2GB) DDR2 PC2-8500C5 TwinX Dual Channel (TWIN2X4096-8500C5C)
which I want to know if it will be ok in a Gigabyte board which is supposedly a PC6400 800MHZ board. Gigabyte GA-G41M-ES2L motherboard.
The 1066MHz. RAM will require a bit more voltage to run than the PC26400. PC28500 is simply PC26400 with the ability to reach higher data transfer rates, but with greater CAS LAT and voltage requirements. The G41 Micro board may not have the necessary voltage regulators or voltage adjustments in BIOS to run PC28500 to specs.
Run the MB through some RAM configurators for tested RAM guaranteed to work.