OK your case NZXT Guardian Black will take an ATX sized motherboard, you have chosen a M-ATX motherboard, the Gigabyte GA-B75M-D3H, a M-ATX is smaller than an ATX motherboard. Compare. This will mean that as your case has a window you are going to notice how small your components look in this big case. So you do not have to worry about not having enough room.
You do not mention budget but I would be looking to getting a minimum of a GTX 660 for gaming though the 650 it will not,perhaps do so well with future games. Have a look here at how these two GPU's compare.
As for your choice of RAM, I have not come across this brand in this country and are you buying 16GB? As this is a waste of cash as 8GB will be more than enough.
You could possibly save a few quid by dropping the wattage of your PSU to 660W which is more than enough for your components.
Are you buying in this country? I get the impression from one of two of your component choices that you may not be in the UK