Well it all depends on how many textures and mods in general you want to use and how much money you want to spend on the pc. If you use only a few texture replacers and let's say, 50 mods in total the 2gb should be enough at 1080p. If you use more mods and want to go all out, then a gtx 780 3gb or even better gtx 780 ti 3gb (or equivalent amd radeon card) would be better.
@ eempc
https://skinflint.co.uk/?cmp=1119923&cmp=1050223#xf_top .. in case you already have the i7-4790, then NO, the i7-4790k is not needed. In case not, then I would say, go get the k-variant as the difference in the price is only 20-30 €/$. But make sure you buy a good cpu fan with 160 tdp minimum, if not better 190 tdp. Both i7-4790 and i7-4790k tend to get a bit hot if the cpu is under 100% load (programs like AIDA64 have such benchmarks). Sure, that will rarely be the case, especially if you only play Skyrim, but well the standard boxed fan would be a little less for gaming with that cpu.
Edit: Ah, didn't see you were looking at i5 versions. Can't say for sure for those. Sorry.