So on my PC im Working on I should get a gtx 680?
What about the CPU? If 720 has 8 cores where the fuck do I match or go better than that
Well, the cell processor also has 8 cores, but I'll be running DmC at twice the framerate with perfect image quality on a i5-2500k (quad) vs. the PS3 version. The GPU and RAM in a home built PC are not just better, but they are also 7 years newer at this point, which is a huge advantage. This outweighs the need for anything crazy like a 8 core processor-- there aren't many of them out and it's just an unnecessary amount of power for gaming (unless you're running a game on a PS3 and need to use all that cell power to avoid overloading the crappy little GPU it has.)
680s are obviously great cards and would scale up for longer than most other cards.
Your processor and GPU choice can't really be sized up yet, but hardware has outpaced the graphical and processing needs of most games. There's no way to predict what the 720/PS4 will do from day 1, but I can't imagine it'll be a night and day difference between them and a good $1000 rig (for the ram, GPU, CPU, and mobo) you can currently buy. The rig might even win. Basically, the more expensive and stronger GPU + CPU you get, the longer your PC will scale up, but there's no way to say for how long exactly...probably a long time, as long as you don't cheap out.