If you are playing on a 1080p monitor / 60Mhz - I would recommend going with the 770GTX 2GB if you want to save some cash (if it makes you feel better and you don't mind spending $60-ish more get the 4GB but you wont need it - more on that in a sec). Not only that, you will be able upcoming games on high/ultra settings with a respectable frame rate for at least the next year.
Ultra settings on BF4 on PC as of right now, are the exact same (if not a tad better) than the PS4/Xbox One version of the game (Here is a video comparison -
http://www.youtube.com/watch?v=jJuz8pCfNJk ) . The 770GTX also is more powerful already than the PS4 and Xbox One's GPUs and they aren't out just yet. Of course, this is an early launch title port- still impressive looking, but the games will start to look better and become more refined/optimized as the years go on.
With the 770 GTX, people are reporting 33 min - 50 avg FPS in BF4 Ultra (1920 x 1080/ MSAA 4X - or 45 - 63 without AA).
As for VRAM - welp, at that Ultra + 4xMSAA VRAM use tops out at 1575 ( Suace:
http://www.guru3d.com/articles_pages/battlefield_4_vga_graphics_performance_benchmark,11.html ) - so 2GB is plenty- it isn't until you move to 2560 x 1440 at the same settings that VRAM use hits 1951 which personally for me would be to close for comfort.
Of course, as you said- you will be doing some heavy skyrim modding- in which case, go for the 4GB. Also for extra peace of mind- people are saying that 3GB/4GB is going to become the new standard like 1536Gb/2gB was for last gen. However, I don't know if that is true but I can believe it.
As for the 780 - again, this largely depends on what you are using monitor wise- but unless you are running 2560 x 1400 or a 120mhz 1080p monitor, I think it is overkill.
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