This depends on your motherboard, and also on the spec already in your system.
Download this program, Its for telling you what your computer contains, and there is a RAM section which will give you the pseed the motherboard supports, and the RAM speed in at the moment (aim for that):
When you run the program, go to the Motherboard button, then to SPD.
It'll tell you the make of RAM and also the speed here. Take note of both as its actually best to get the same make too on some models. If not possible though, aim for the speed given.
Mine for example is PC3200 which is rather standard nowadays, and is also named DDR400.
i hope this helps