This depends on the RAM and motherboard, in a perfect world where both RAM sticks and the motherboard you have are perfectly compatable, they should work fine. But since there are about a thousand + combinations of the 3 that are possible, the RAM can very well be the issue.
Does the PC run fine with one one of the sticks installed? If it does, you have your answer. Get a matching RAM stick (for the fater RAM of course).