This is what I do when I get bored.
I have 2 GB of Muskin XP2 8500 Ram (1066 Mhz) installed in my first PC and bought 2 GB of Crucial Ballistix 6400 (800 Mhz) for my second build. Seeing that they both use the Micron D9 (GMH) chip I thought I would try and see if they would work in Dual channel together. Guess what they do. Took them right up to 1000 Mhz together with no problem. All done at 2.2v. They would not boot together at 1066 though. The Crucial will boot by itself to 1100+ Mhz @2.2v and the Mushkin by itself will go 1200+ Mhz@ 2.3v. Still 1000 Mhz mixed is pretty good IMO. Just had to loosen the timings to the Crucial timings. You should always buy the same brand and same cas but I thought I would test it out and see.