Oh, and can I overclock that card with that board? Or does the board even matter.
The i3 can't really overclock, so it's a moot point. That board is a z77 chipset, which is for overclocking, and it's also a micro-atx form factor, which means you're going to have space issues. (And it'll look silly in a big case.) Get a board that uses the h77 chipset, and is a full atx board.
I wouldn't use an external especially for games. I don't think you'd be able to put a lot of games in that 120gb SSD especially since newer games are taking up so much space. I would suggest a 1b HDD which is on sale right now.
The reason I ask is because I am also planning a build. I was considering an I5-3570K and figured out I have an aunt that lives near a Microcenter in NY and could have her pick up the part for me and ship it to me in Florida. But if I could get the $40 off on a MOBO as well i might as well get that too from microcenter instead of say newegg.