first of all, with ENBseries 0.229(a must have for skyrim), you will get dx10 type effects and ambient occlusion, smaa, water effects, lighting and shadow effects etc., all sorts of fun stuff.... you wont need an nvidia card for that. second... skyrim will crash when it hits 3.1gb vram... anything over that wont help you, but the enb will make sure you dont cross that barrier if you have a card with more than 3gb vram as the game engine will try to use it and fail. third and most important.... and you can go on nexus forums to confirm it if you want or dont believe me... but 2gb cards will not cut it if you want to run ultra maxed texture mesh mods... everybody over there ditched their 2gb 670/680's for much more stable 4gb models or 3gb 7950/7970's because fps falls through the floor as soon as you use up the vram which is very easy to do.
3gb and up for skyrim... its the only way to go if your serious about your skyrim experience. if you get an nvidia card, make sure is not some wimpy 2gb model.