Here's a wiki on memory interleaving, but to directly answer your question; yes, mixing memory types/speeds/etc will result in slower memory timing speeds (lowest common denominator) and certain functions being disabled if memory sizes are mixed. The reason for this is that memory interleaving is a sort of 'computer math trick' that allows the RAM to have faster throughput if all of the memory address' align (which means the sticks would have to be at least the same sizes). Your motherboard/chipset combination will determine what your limits are based on the memory types you put in, in other words, memory functionality (dual-channel/interleaving) is a function of the motherboard/chipset you use and not necessarily of the RAM itself. If you have 2 sticks of 4GB (2x4GB) in slots 1 and 3 (usually how it goes in dual channel) they should be of the same type (speed/latency/etc.) otherwise the RAM won't 'work' at all (won't be recognized) or the RAM will 'run' at the lowest speed of the two. Mixing the sizes in the banks (2x2GB and 2x4GB) should keep the 'dual-channel' configuration if that's an option in your chipset and the dual sticks are the same (i.e. speed), but it could depend if you will still have memory interleaving as that is an algorithm employed by the chipset manufacturer which uses 'banks of memory' (could be the entire stick or a few chunks of it).
In the long run this might not affect you depending on what you do. If you're only ever doing basic (not video/games) web surfing, and document editing, even some power stuff may not take a hit from this. If, however, you're a gamer, like watching a lot of videos, do any sort of virtualization, are an enthusiast, etc. etc. memory interleaving 'could' make a significant difference to you. I say 'could' because with any build there are a lot of factors that determine your performance but RAM and HDD are usually the first things to get upgraded.
You can use a tool called CPUz to check out what your computer is reporting 'up-the-chain' if you will (i.e. from the BIOS to the OS) about your hardware configuration (including your RAM).
Hope that helps
is this valid even if I mix in pairs (2x2GB + 2x4GB)? And I am going run some virtual PC wit Linux on Win7x64 and some development tools separately or maybe in parallel, like Visual Studio 2010, Delphi... The question then is transformed in nest: What is better? 2x4GB OR 2x2GB+2x4GB? – ALZ – 2013-12-15T22:03:47.590
I've edited my comment to better reflect. TLDR: it depends :( .. you can grab CPUz from the link and test it out. You'll have a total of 12GB of RAM at your disposal, as long as it's fast enough (clock timing and CAS/RAS) you should be more than good to have a Win7x64 development machine with some Linux VM's (my personal laptop is Win7x64 dev machine with 2 VM's regularly running hosting my development environments..total RAM: 16GB, average ever used: 4GB, unless I'm playing a game too) – txtechhelp – 2013-12-16T23:42:15.703
So...if I understand correctly, if you're adding a second stick of RAM to a machine that otherwise only had one stick, your performance can only get better, since you didn't have interleaving to begin with (only had one stick of RAM). But: Having interleaving might make that second stick of RAM have an even larger benefit than you would get if you just replaced the original stick with a larger size. – mlissner – 2014-05-01T21:27:22.373
@mlissner, yes, kind of, it is a possibility you could get a 'double' boost (so to speak) in that scenario; just remember that your mileage may vary, i.e. just because you added some RAM doesn't necessarily mean your programs/files will load faster, but your videos 'might' play smoother or games 'might' not stutter as much, it will depend on other factors of your system as well. – txtechhelp – 2014-05-02T00:37:16.503