hi eski_komut,
since you have four slots in ur main board, it seems to be a dual channel main board. But whatever may be the case, for two rams to work together on a main board, they must match. ie, their frequencies, type of memory and everything should match. It's better if both rams are from the same manufacturer.
For example, the frequency of one of them might be 333 MHz and that of the other may be 533 MHz. If that's the case, then they won't go together and you will have to change any one. There are various other parameters which must also match when using more than one ram in a board. See if everything matches, and if they match and still they can't be used together, then there's a problem with your board.

hey silver scorpion as far as i know, the frequencies of two rams need not always match for both to be used together. i have friends using 533MHz and 667MHz rams side by side and their system works fine. i even have a friend who uses a 266MHz, 128mb and a 400MHz, 512mb ram simulataneously and his system too works fine. Actually motherboard manufacturers specify their mainboard memory compatibility issues about the restrictions when using multiple memories, eski u can check out the datasheets for ur mainboard on the net for furthur details or specify them here so that ceans can check it out for u.