What could would you advise to handle the latest texture mods, ENB configurations, optimum preferences settings, etc.?

I'm looking at 2 options mostly.

Would (1) GTX 680 4GB be enough? - prefered

Or will (2) GTX 680 2GB SLI?
I suppose at that price, I could consider (1) GTX 690.

Any help or a little education would be much appreciated!

..... I'm assuming your serious... Any of those would be absolute overkill. If your processor is strong enough you could probably have several instances of skyrim playing all at the same time stretched across 3 monitors and still have playable frame rate.

So in short, yes any of the choices you mentioned will run skyrim with all of the greatest texture mods, with shader mods, with anything else you can imagine in the future of the games run and probably not even break a sweat.

I doubt you actually need 4gb of Vram, most people seem to run texture mods with 2gb. You can actually run most texture mods with a lower-end card, the only problem is that the lack of VRAM on 1GB cards will cause your game to crash immediately if you ever step into an area that is loading a lot of hi-res textures at once. For this reason i could run WATER and 2K Textures at the same time on a 460M with barely any slowdown, but as soon as i got into a busy area I would crash instantly. I would recommend a card with 2GB of VRAM, but as Charon said above more than that is probably overkill.

Okay, thanks. Would you recommend a cheaper card then?

It depends at what resolution you're playing at.and what the other specs are on your box.

as an example I run with:

i7-920 @4GHz
25GB Tri-channell DDR ram @ 2000MHz
Win 7 64-bit installed on SSD
Skyrim installed on SSD
HD7970 XFX Black Edition

And I can run on Ultra with pretty much all HD texture/lod/mesh replacers out there, SMAA+2xMSAA shoadows on everything + ENB with SSAO/SSIL and Dof on max quality at 1080p with a 45 to 55 fps

But when I try to run on 2560x1600 with SSAO/SSIL and Dof on lowest quality setting SMAA, no MSAA, plus all the other settings as 1080p, I'm lucky to hit 35 fps.

I can verify the difference between 1Gb VRAM and 2Gb VRAM makes a big difference in gameplay. The way I understood it, running two 2Gb Memory Video Cards together does not increase the total V-RAM to 4Gb, I would go with the newest card you can afford for future-proofing.

well are you planning any other parts? like Processor, ram, motherboard, power supply? If so i would devote some of that cash flow to a better version of them if possible. A single GTX 680 2gb would be plenty.