I've been thinking about upgrading my PC for a while (as you can see in my specs, some parts of it are pitiful!) and I ended up at the conclusion that my ideal GPU would be an ATi HD7850. I then thought about the possibility of getting an MSI Twin Frozr HD7950, but dismissed the idea due to two things - one being size constrictions and the other being a possible bottleneck.
So firstly, in my CM Storm Scout would I be able to realistically fit a GPU like the HD7950 into the case or am I forever limited to something like the 7850/7870 (which I wouldn't mind at all!)
And if I can fit a 7950 into the case, is my future system a bottleneck (I don't think so, just wanted to check!):
Phenom II X4 @ 4GHZ - 8GB 1600MHZ DDR3 - XFX 850W PSU - 500GB HDD - ASUS AM3+ MOBO
No way! You have a really nice system by the way. You are nowhere near bottlenecking! The size is a different matter though, the 7950 is 10.28" x 4.37" x 1.5" so just open the side of your case and measure it to see if it will fit.