When you are mixing memory by brand alone one thing to do is look over the specifications to see what the latency is for one or the other. That will help if you are adding ram to what is already installed. Even with that being the same some make boards will strongly favor certain brands of memory over others for some reason. Asus is noted to favor brands like Kingston, Corsair, Crucial, Mushkin, and a few others.
With two 512mb dimms you would want to utilize the dual channel mode unless you have other reasons to keep that disabled. If the Samsung in A1 and Seagate in B1 configuration fails you could try reversing to Seagate in A1 and Samsung in B1 there. If both configurations see problems then try each dimm alone to see which is the best to go with.