Being a AMD fanboy my suggestions are as follows:
With your current listed specs, your biggest bottleneck is going to be your hard drive. I would suggest getting 2 500gb hard drives to run RAID 0 or a SSD for your OS, 1tb+ drives are better for data storage. Next I have a personal preferance for nVidia graphics cards. Keep in mind this is just my opinion and I'm not saying an nVidia card is going to be better than an ATI card. I just like cuda/sli better. Depending on how long you want to be gaming for, I would even recommend dual graphics cards (GTX 760 is what I'd suggest). Next whatever you do with the RAM go dual channel. I would suggest either 2x8 or 4x4 to get that 16gb (go to 32gb if you can swing it) also if you're gaming go with the ASUS Crosshair V mobo. Lastly be sure to get a modular power supply and you may want a little more than 520W, I'd suggest something more like 700W-1000W, but I highly recommend it to be modular. Lastly, not sure why you need 2 cases.
Don't forget if you're gaming, do not skip out on the mouse and keyboard. I like the Razer Oroborus set, but I personally have a razer naga (5 finger discount at Walmart) and I'm not sure which keyboard... pretty much though you can't go wrong with a razer one.
Hope that helps, just keep in mind that these are just suggestions and I have no idea what price range you're looking at.