Just on the graphics question: It's hard to give advice for sure without knowing what resolution you're looking at. I assume thought that you're talking one 1080p @ 120 Hz (that is, 3D) monitor. If that's right, then yes, two 670s will absolutely make a difference compared to one, especially if you're talking about a game like Crysis 2. Here's an example (from a review of a souped-up 680 but which shows 670s on very recent drivers): http://www.anandtech.com/show/6096/evga-geforce-gtx-680-classified-review/5
You can see that a single 670 won't even come close to 120 fps on the most recent and most demanding titles, even on 1080p. In fact it may not even get to 60 fps, which means SLI won't get to 120. But yeah, if it's for 3D gaming (and/or you want 100+ frames on 120 Hz without 3D) and you want decent framerates on stuff like Crysis or BF3, then definitely you'll need two cards.