1. No unless you're overclocking, where some motherboards will overclock better and some won't overclock at all, you pay for the features on the motherboard.
2. You can mix it up, though i don't think you'll ever be sure that it will work. Also think they need to be running at the same speed, and i can already figure the 2x2GB's are pretty slow.
3. Nope, but you'll be able to find loads of guides online, and videos that guide you on youtube.
Hope this helped you!