Regarding visual/graphic mods: it entirely depends on the performance impact it can have from system to system and mod list to mod list so I can't give a generalized assessment there.
On VRAM: At that resolution 3-4gb is the sweet spot really, unless you plan to use a lot of graphic mods like 4k textures, higher def poly mesh replacers, etc.. imo really 4k @2k gaming is still just a pretty waste for me, some will claim that "oh there's a difference still" etc.. but really the difference is quite a subtle one and these people probably play with their monitors real close to their eyes, unless you're using some form of downsampling using overly high res textures above your monitors max pixel resolution isn't the best lod(level of detail):perfomance ratio you'll get. If you're able to get something like a card with 6gb of vram; my personal suggestion would be to just get a 4gb one really, but if you are able to get a 6gb one try and just stick to that unless the card you're getting is like a titan x or a 980 ti, problem with cards with 8gb of vram that are even a bit below the tier end consumer base of those two cards right now is 95% of them I can guarantee you will start chug on the power of the card itself when you're utilizing 80-90% of the vram usage of the 8gb; this bottleneck on the vram size:gpu performance is well know as vram increases to like some whatever niche amount nvidia or amd puts into their semi high end cards. Then again it will also probably heavily depend again on how heavy the 3d screen rendering of a 3d application to make the gpu get bottlnecked by its horsepower when you use like 7gb/8gb of your vram. I don't have much experience with cards on the range of 6gb so I can't tell you if I can make the same assessment but with a little research you can quickly find the gpu power:vram capping ratio of that card and see if u can really make use of even 5gb of vram in most scenarios if not then I'd say go for a 4gb one really since the extra you pay for even a 6gb version won't really hold up to how well gpus on the market as of this moment will perform on a near 80-90%vram usage, especially if you expect to run 3d applications with standard or the most latest post processing effects at max or for instance Skyrim use enbs.
If you max/cap out on you gpu's vram, ram/hdd texture file swapping which is a a magnitude than what a gpu's ram/bandwidth speed is (depending on the system memory/storage drives you have ofc) Also mesh assets also contribute to vram/ram usage
Regarding game recording:
Best suggestion I can give really is to just go for a game capture card if you have a setup to accommodate it, performance impact is near 0- negligible whilst recording with a dedicated screen capture card.
Edit : If you plan on screen recording using a software solution then try and get either a dedicated storage drive to write capture media in (ssds would be the best choice a hdd will work very much quite well as well in reducing the performance impact whilst recording and playing as long as you record to a dedicated drive really.)
General recommendations/suggestions:
As of the moment of writing for Skyrim at least and a lot of other games is get an Intel cpu, I use AMD myself but in the near future I'm definitely going to be switching back to Intel.
Disk performance ie. SSDs/Raid setups will help a lot in performance in just about any system experience.
My specs:
FX 9350 - 8GB RAM - R9 290X
I don't use much graphic/visual mods in my normal playthroughs really so I don't think my mod setup/play setup will provide any useful benchmarks for you; plus I usually play at 1280x800 for just games I know I'm weird. It's mostly because I want a constant 59 framerate for Skyrim and a lot of games for instance and I don't at all mind the resolution and such, also I run a lot of background applications that hog some cpu and vram usage 50-70% of the time so this kind of setup is ideal to me.
last I played this on a higher was; iirc I get around 45-50 ( using a limiter at 50fps) in the most heaviest of mod setups I have at 2560x1440p