You have a nice build there for the budget, but if I were you I still wouldn't expect some of those games to run at full speed all the time. You'll never know how intense the emulation can get and having the expectation of 100% speed all the time will disappoint you. Depends on your settings too, if you're a graphics whore like me then you'll love to have max/close to max GFX settings and that may cause hiccups too.
To address your question about the case size, the motherboard you will get has an ATX form factor, so any case that is ATX Mid and up would be fine. Since your PSU is not modular you should keep that in mind as you pick a case. You'll be shoving a bunch of excess cords either behind the case or in a corner of the case. My last build I had the same situation with a Corsair GS600. All the extra cords took up a lot of space, and as a result they slightly obstructed ventilation and picked up a lot of dust as well.
If you want to look at some modular PSUs, here are a couple at around the same price range and in the 600W range as well:
Corsair CX600M: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139048
Raidmax RX-635AP: http://www.newegg.com/Product/Product.aspx?Item=N82E16817152046
To address your question about the case size, the motherboard you will get has an ATX form factor, so any case that is ATX Mid and up would be fine. Since your PSU is not modular you should keep that in mind as you pick a case. You'll be shoving a bunch of excess cords either behind the case or in a corner of the case. My last build I had the same situation with a Corsair GS600. All the extra cords took up a lot of space, and as a result they slightly obstructed ventilation and picked up a lot of dust as well.
If you want to look at some modular PSUs, here are a couple at around the same price range and in the 600W range as well:
Corsair CX600M: http://www.newegg.com/Product/Product.aspx?Item=N82E16817139048
Raidmax RX-635AP: http://www.newegg.com/Product/Product.aspx?Item=N82E16817152046