Ok cool let me clear some things up then before I build for you:
Windows 8.1 has a sloppier UI than Windows 7, its not as bad as Windows 8 but eh thats not that hard to do. Benchmarks have show that Windows 8.1 is faster in most processes including gaming. Its only a couple of frames per second faster in gaming on average but hey, with a build of this budget, you will have frames to spare. So its your call on that front.
That case seems fine if you want to salvage it, but there are a couple concerns. Most mobos will come with front USB 3.0 headers, which your case does not. Shouldnt be that much of an issue, you should still have two functioning USB 2.0 ports on the front of the case. Im assuming the case fits an ATX boar dand can house modern graphics cards. Its quite possible it was never made for a large three fan graphics card like a 970. If you do not want to get a new case its just a risk youre going to have to take. If the case ends up not working out youre going to have to buy a new one and your build will be staled a week or two. I can suggest a modest case that is similar in design but features more functionality if you would like, it would have a minimal effect of the performance of your build.
So you arent interested in overclocking now, is it something you would like for the future or do you want me to just rule it out. It might only save you 30 bucks to take out overclocking capacity.
SLI and Crossfirex are proprietary technology that allow Nvidia and AMD graphics cards, respectively, to run multiple GPU sets at a time. That is, you can run mutlipe graphics cards for more power than a single graphics card. The main benefit of this technology is that it gives you the option, in the future when you want to upgrade, to buy a second graphics card of what you already have (which should be cheap since its older) and get the same if not more power than just replacing the graphics card with the latest most expensive graphics card. For Nvidia cards this technology is commonly tied to Z chipsets on intel boards so if you dont plan on overclocking its assumed you dont plan on SLI.