there are very new mobos that use the new DDR2.

You must use two sticks of the same memory capacity and and ,to be safe, timing [CL] & the same mfr's chips on the stick.

Many places sell them in sets, which implys some kind of special matching, which is really not needed. But when they sell them in pairs they usually knock a few bucks of the separate price times two [really ONLY a couple of bucks.LOL] Somtimes the higher speed ones are lower price. If so buy them. Won't run any faster but will give you more memory timing margin.