1. Perfect. Yes, no issues upgrading with Win7 x64.
2. Iffy. It's ram. There's only 1 guarantee about ram. There are no guarantees. I've used 2x sticks of ram, both supplied from the same vendor, both identical color, model, size, speed. The only difference at all was the batch was 10 numbers different, in other words I got 1 stick and the other was made literally 9 sticks later. And they would not play nice under any circumstances or changes to settings. Totally incompatible. I've also used 2 totally different sticks, from 2 different vendors, where the only thing the same was the 1.65v rating. And they worked perfectly, no adjustments needed.
Mixing ram is always, always a crap shoot. It's impossible to answer if they'll work together at first, or if they'll need adjustments or if they won't work at all. It's always recommended that you get whatever you want, in 1 kit. Those sticks are factory tested and guaranteed to work at rated speeds.
I will say if you are using Kingston HyperX, don't add a different vendor, chances are good they'll not play nice so stick with Kingston if that's what you have. If you have Corsair or G-skill, there's a good chance a different brand will work, those sticks are quite forgiving, just don't add Kingston hyperX.