Oh by the way, I didn't notice this earlier, but the motherboard you have is mATX. That's a good thing because it gives you more options with your case. You can either go with a Mid tower case (such as all the ones I've linked above) or a mATX sized case such as:
http://www.amazon.com/Fractal-Design-Cases-Black-FD-CA-CORE-1000-BL/dp/B0074V2UXI/ref=sr_1_2?ie=UTF8&qid=1366753418&sr=8-2&keywords=mATX+case (very solid case, only downside is that it has a top mounted power supply). Since the expand-ability is limited by the motherboard rather than the case, there's no real reason to go for a Mid Tower over an mATX case, but mATX cases are considerably smaller which is always nice.
500W-600W is going to be able to power any single GPU configurations (note by this I mean a single physical graphics card, the number of slots is irrelevant if that clears up any confusion). As graphics technology progresses, the power efficiency shoots up, which is why you can be fairly certain that with a decent 500W supply you'll always be fine, just as in a couple of years time 400W will likely be the standard. 500W can accommodate any current mainstream graphics card on the market really (unless you're heavily overclocking with something like a 7970).
Most every graphics card that runs modern games decently is a two slotter. Any mATX/Mid Tower will be able to accommodate this without issue. You might be meaning SLi/CrossFire though, which the motherboard doesn't support (not that it needs to, these are features for people at the very extreme end of computer gaming, i.e. multiple monitors and so on). This is when you have more than one GPU, but you shouldn't have to worry about that at all.
The only requirements to run a graphics card better than the one you currently have is a big enough power supply. As I said earlier, 500-600W will take care of anything currently on the market with ease.
If possible you might want to return the 8gb of RAM and get 2x4gb sticks, or buy another identical 8gb stick. I only say this because having 2 sticks rather than 1 allows for dual channel mode, which doubles the bandwidth of the memory. This is important for games such as Minecraft, which are CPU/Memory bandwidth rather than GPU bound.
M