You are both right. If I plan on SLI then yeah I should go with nvidia, but my concern is that I didn't plan on going SLI/xfire right off the bat. The 7970 seems like the better single card. If this single card could push the 1440p display fine on it's own then I wouldn't have a problem. There are 4gb 670 options that I've seen, but what I've also been reading is that some demanding games don't even push the AMD cards beyond 2gb vram.I'd never go back to 1080. There are 120hz IPS monitors (http://overlordcomputer.com/) but there are some caveats. As for the video card, if you plan on throwing a second card, go with nvidia (670), if you're staying single card go with ati (7970).
I guess what it comes down to is that I don't feel that a single 670 can push a 1440p display properly on it's own and I would be selling myself short by doing just that. The 680 is better, but for a premium price and a little 5% (or something) increase in performance it isn't worth it. So it seems either get the 7970 and be done with it or get 2x 670's.
Feel free to correct me on any point. I'm trying to finalize these last few details and as you can see it's a bit frustrating.