First time to build need advice

you need a z77 lga 1155 motherboard
oem windows disk
cod games have been using the same graphics engine for 4-5 years now so they are not demanding at all. you will have no problem maxing it out with 7870

i have an i5-3570k no after market and it runs BL2 fine (i do have a 670 but still), my friend has a 7850 and he runs BL2 well too (using BL2 as a benchmark) (he has an AMD 8 core (fx-8 8150) what ever that means)

and i'm sure you have a CD drive somewhere

if you can have a direct link to your router (Ethernet) then   yay, if not consider getting a wifi card (i prefer a pci but usb is an option)

your main board, mother board, MOBO what ever people call it these days is fine (if you'll never sli which is fine)

with your case you might want to consider getting something with usb 3.0 (avoid the need to change your case and wasting usb 3.0 headers (only for 2 ports but still)
another option is to get one of those usb header things that sit in one of your bay drive things