Hi!
Well, first of all what type of RAM are we talking about? DDR3, DDR2, ect.
Also, is your motherboard single/dual/triple/quad channel memory compatible?
If you have DDR3 and your mobo supports dual channel, then you will get the best performance gain if you buy a dual channel DIMM. (Going by the assumption that your current DIMM is also dual channel. If not hten 2 dual channel DIMM's are required...)