You are right about mixing different RAM modules — if there’s one thing you absolutely can’t mix, it’s DDR with DDR2, or DDR2 with DDR3 and so on (they won’t even fit in the same slots). RAM is pretty complicated, but there are a few things you can mix and a few things you shouldn’t. In any case, I don’t recommend it. If you’re buying new RAM, you’re probably going to make your life a lot easier by buying the exact same model as you currently have in your machine. That said, if you absolutely have to mix them, here are some general guidelines.
You want to make sure each stick has the same cas latency, timings and voltage. While you can tweak these settings in the BIOS to make the two sticks match (we briefly showed you how when we discussed overclocking your processor), I don’t know why you’d want to. You’d have to spend quite a bit of time messing around with it all and probably underclock at least one of your DIMMs in the process — and there’s no guarantee it would work even then. Really, it’s a lot more trouble than its worth.
Mixing RAM speed, however, is a slightly different matter. In theory, if you had to, you could mix, say, this Patriot model (DDR3 1333) with this Corsair model (DDR3 1600), since they have the same cas latency, timings and recommended voltage. Your motherboard would probably just automatically underclock the faster one and you wouldn’t run into any problems. Thus, it’s possible, but note that when you start mixing speeds, a chain is only as strong as its weakest link, so to speak. Your RAM will only run at the speed of the slowest DIMM, unless you wanted to overclock the others.