Various brand names that are reputable include Corsair, Mushkin, Crucial, and Kingston. If possible, find out what memory is currently in his system and get the same brand. Stability problems are less likely with similar or identical DIMMs. If it isn't possible, then it's not a big deal. Most motherboards have no trouble mixing and matching all kinds of DIMMs. Choose whichever brand name manufacturer is the cheapest if you are unable to find a matching DIMM.