1. I will have 2 4 GB sticks of DDR3 1866 RAM that I bought for the new PC, and I have several 1 GB DDR3 1066 sticks left over from my old PC, and there are 4 RAM slots on my motherboard. It's generally a bad idea to mix RAM of different speeds right? But for future upgrades is it okay to mix different memory sizes, i.e. if I buy a couple 8 GB of 1866 later on, would that be fine?
Don't put your old RAM sticks in there - you won't need the extra 2GB you'd get and it would slow your other 8GB sticks down to 1066MHz. You can install more 1866MHz sticks later just fine.
2. I bought an SSD, but I plan to re-use my HDD as well. Currently my HDD has Windows Vista on it, but I will be installing Windows 8 on the new PC. I'm guessing I will want to reformat the HDD for long term use, but would I be able to mount it as is in the short term, so that I can retrieve all of my files from it before doing a reformat? I did not make a backup, so I need some way to get the data off that drive.
First things first, only have your SSD in your PC to begin with and no other hard drives. Once you've got Windows 8 installed, open your PC up and fit any other drives, and in Windows you should then be able to access your old one so you can back up your stuff. Once you're done, format it and you're good to go.
3. I've never used an SSD before, but mine will definitely not have enough capacity to store everything. Obviously I would want my OS on the SSD, and a handful of games I am currently playing, while the bulk of my storage would go on the HDD. Is there anything else I would definitely want to put on the SDD, or anything that I really shouldn't put on there?
Just the programs and games you use all the time. Everything else can just go on your storage drive (music/documents/pictures/downloads/other games and so on).
4. Along with the previous question, if I'm only storing a small portion of my games library on my SSD at any given time, about how long does it take to move games on and off of the SSD? Edit: I realize that question was a bit too generic to have a specific answer, so assume the games in question are 10 GB.
Not that long really, it would be minutes - maybe 10 if that.