Hi! A few more specs would be nice. Is it SDRAM, or DDR? How many memory slots in the motherboard and is it a different brand or timing from the other memory?
Some motherboards will only accept certain configurations; such as double-sided ram in the first two slots then either one double-sided in the third or two single-sided chips in the third and fourth.
Two different brands of memory can work together if they are the same speed, however there is the chance that they will not work together. This can occur with the same brand memory purchased months apart (this has happened to me personally). For the sake of keeping problems to a minimum I always suggest my customers purchase their memory chips at the same time if they can afford it.
Hope this helps you,
Jeff