For best compatibility, you should try to get the same brand of RAM if you wantto run in Daul Channel. However it's possible to mix RAM's. It's almost like mixing battery brands, the manufacturer tells you it's harmful and can cause problems...but 9 times out of 10 it works without any problems.
You do however have to make sure you get the same speed RAm, and preferably same timings/voltage settings for both. If you have one stick of PC3200 that will do 2-2-2-5 @ 2.6v, and another PC3200 that will only do 3-3-3-5 @ 2.7v, you'll have to run them BOTH at the slowest settings, which hurts performance.
