It's not worth it, unless as BigMack70 said, you're gaming at resolutions above 1440p.
Also performance/value, the 670 gtx is better, as for the 670 gtx with 680 gtx parts, you mean the pcb of the 680 gtx, it's the 670 gtx EVGA FTW and FTW Signature 2.
No there's not. I went from a 300+ mod install with a config of higher than ultra texture/etc settings and of ugrids=11 down to a just slightly tweaked ini file + maybe a dozen or two of the more important mods (including all the best texture packs).
Know what happens? In both cases, my 7970s sit there completely maxed out at 3GB vram. Know what else happens? Zero performance difference between the two, even though the first setup was vastly more demanding on the vram.
Games, especially something like modded skyrim, will fill up vast amounts more vram than they actually need, and there is no visible performance difference as long as you stay above some "required" threshold. Figuring out what that threshold is is a very tricky thing, but from forums etc it's pretty clear that 2GB has you covered for Skyrim with as many mods as you want.
Yes, but imagine having a card with a Vram of 1GB. You were maxed out at 3GB anyway. I used to have a 460 GTX 1GB until christmas, and next to upgrading, I'm sure the extra Vram did something on its own. But then again, it was 1GB, likely below the threshold you are talking about.
Btw, I noticed you also have a Noctua NH-D14 as do I. What are your arguments for having that CPU cooler over than, say, a water cooler? Just want some knowledge