CPU vs RAM

It will automatically clock down to 1066MHz like the RAM you already have.

The Athlon II CPUs can use either DDR2 at up to 1066 MHz or DDR3 at up to 1333 MHz. DDR2 doesn't come in speeds faster than 1200 MHz, so if you have memory rated for 1333 MHz, it must be DDR3. Your CPU will thus run your DDR3-1333 memory at its full rated speed, so you are perfectly fine.

Crk, you need to check your system specs and the RAM you got to make sure you have the right type of RAM for your motherboard. If you have DDR2 now, you will not be able to mix it with DDR3.

He has DDR3 RAM since he said the memory runs at 1333 MHz. If his RAM will fit in the motherboard, it will work since DDR3 won't fit in DDR2 slots and vice-versa. If he does not already have a board, he needs to get an AM3 and not AM2 or AM2+ motherboard if he wants to use that DDR3-1333 he has.

I made the mistake of assuming that the OP already knew that the RAM would fit. Since no mainboard was listed, it is unclear whether he has DDR2 or DDR3 installed.

Crk -- what mainboard do you have?

Leaps-from-Shadows . Thank you for the answer. "It will automatically clock down to 1066MHz ". Which is exactly what I think it will do.
Since you guys were nice enough to give me some answers, I will not leave you in the dark. I have a very old PC. I am traying to bild another one. I allready got CPU and a friend gaved me the ram for free. The rest of the components will come after: MB, HD, video card etc.....
MU_Engineer ,thank you alsow for taking the time to answer to my question, but I think you are wrong. As you can see here : http://www.amd.com/us/products/desktop/processors/athlo...
AMD Athlon X2 250 only supports a maximum of DDR3-1066MHz.
P.S: The motherboard that I can afford to buy is a gigabyte that supports DDR3-1600MHz RAM. No problem there ^.^

Ah, that's right, the Athlon II X2 250 (not the 250e) and the X2 220 are the two Athlon IIs that are limited to DDR3-1066 memory speeds. All of the rest of the Athlon IIs can use DDR3-1333, according to AMD's "Family 10h Desktop Processor Power and Thermal Data Sheet, #43375." If he used DDR3-1333 with that unit, it would clock itself down to DDR3-1066.