Sounds like you have a pretty good idea of what you want to do in the next year or so with this build then. I'll definitely vouch for the p67 extreme 4 gen.3. I built on it and haven't had any issues with it at all (no fryings like EVGA, no coldboot issues like Asus). Looks sharp in the case as well.
By the time Christmas rolls around, there should be some real competition in the 350 dollar range with 28nm tech (think cards faster than the 7870, which is likely to price-drop here in the wake of its performance vice previous gen cards). As far as going from an i5-2500k to an i7, you'll just have to decide if you do enough tasks that benefit from hyperthreading
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Otherwise you could potentially just go balls to the wall with a big 600 dollar monster GPU! In all seriousness though, you sound like you know what you want to do. Make sure your PSU will have enough headroom down the road (the lower power consumption from 28nm cards so far means that a 550 will have some legs, but a 650 ultimately is safer if you can find a reputable 650 with the connections that you want and a single 12v rail prior to purchasing the Antec. Good luck with your build chief. Should come out nice.