to answer your questions;
EVGA's like the ROG series mobo's are built for overclocking. EVGA's mobo has the voltage readpoingts and the built-in voltimeter. some goodies and the better quality components(meaning stable OC's and longer OC'ing sessions) plus the dual lan.
The other 2 boards from ASrock; one's a Z68 chip board and the other is a P67 board.
There is a thread on tom's
here - read it and it'll explain the benefits of a Z68 board. Thus the slight price premium for the ASrock Z68 board.
If your not interested int he Z68 (in that matter an SSD cacheing files) then the P67 is the route to go.
stay away from the 6670 - its a good htpc card but a sitting duck when it comes to helping with games and offloading rendering duty.
GPU recommendation - don't go below a GTX560Ti or a 6870 for your duties and gaming. A lil higher to pay for but they will give you a good headroom with gaming and work. If your still unsure of spending that much - get a 5770 or a GTX550Ti
Thanks for the compliment but we're not outta the woods yet.