I don't think R* is saying it'll require a 680 or 6-core CPU to max out, just that it can take advantage of the extra resources. Most games these days only support 4 threads, so if it can actually use 6 or 8, that's good news.
Considering only a dual core and 8600GT is required at the bare minimum it seems this should be fairly optimized, but we'll see I guess. LA Noire was a disaster on PC.
What R* game wasn't horrible on the PC recently? Hopefully they improve their track record here. Max Payne was always a PC-centered experience/game so hopefully it remained that way (I'm not aware of the Max Payne 3 dev process but they probably focused on console first )
The art style is clearly GTA-ish, but the game mostly stays the same. Which is good. R* understood that somethings just are nice they way they are, and didn't take the risk of updating the gameplay to modern standards (by the most part).
So if you liked 1 and 2, you'll probably feel right at home with this one. (from what I could gather)