I'm thinking of getting 64 bit W7. I currently run 4gb Gskill Ripjaws 1600 9-9-9-24 timings. If I were to get 64 bit and another 8 gbs of RAM (2x4gb 1600s) can I use the 8gbs in, say, slots 1&3 and the 4gbs in slots 2 & 4 (or whatever slot combination therein)? I just wondering and I have the mobo manual at home but I'm at work right now. Thanks.
It's never recommended to mix RAM as it often will not work. It's even more unlikely that mixing RAM of different size DIMMs will work. 8 GB. of RAM is plenty for most folks unless you are running large or many applications.
The only way to tell if mixing RAM will work is to physically install it and see, though I don't recommend mixing at all, exspecially with DDR3 which is more sensitive due to the higher frequency.