If im going to build a pc and use an env with skyrim what causes the fps to lower and how do you counteract it other than excellent graphics card

Well the two main aspects that affect frame per second are the CPU and GPU. With a heavier emphasis on GPU regarding Skyrim.

Beyond those two aspects a solid-state hard drive is a big factor on overall performance when it comes to reducing shuttering in game. RAM is important and 8 gigs should be sufficient.

ENB performance for the most part dictated is by the GPU performance. Better GPU equals better ENB performance.

Assuming top graphic card is an intel i5-4440 quadcore sufficient for solid fps with othee mods such as interesting npcs and falskaar

I couldn't find any benchmarks using Skyrim intel i5-4440. That processor is sufficient on the CPU side. The GPU or in this case the correct term APU a processor that does that combines the ability to do CPU and GPU calculations. The i5-4440 has HD Graphics 4600 processing unit. Check thishttps://www.youtube.com/watch?v=CS0reJ_F3uI all though it is tested on i7 4770k 4600 hd but performance wise you will hardly notice any difference as they share the same HD Graphics 4600.
another https://www.youtube.com/watch?v=MVbwsPTNiqY with a slightly different CPU but same graphics unit.

All that being said I think you'll be able to run Skyrim with decent settings although not maxed out. I don't see a problem with you running falskaar but interesting NPC's might cause frame rate drops. You will have to see for yourself. ENB is out of the question except run sum of its performance enhancing options. You might get even better performance optimizing the textures which you can Google to find out more information.

I'm running Skyrim on HD 4400 graphics, and the high res textures didn't noticeably impact my FPS. I find that lighting and associated shader effects are what kills frame rates, rather than merely pushing pixels.

Excellent thanks for the insight! what kind of frame rates are you getting out of curiosity?

Average 15 FPS outdoors, 25 indoors. Frame rate can drop to 15 indoors when in combat with lots of actors, and occasionally tanks to 10 when there's a lot of dynamic lights and/or shader effects on screen. I consider 25 to be fairly smooth, 15 to be playable, and 10 to be jerky.

Note that those numbers are very thermally-limited: I use a Sony Vaio Pro, and if I put it in a colder room or a draught (draft for Americans), I get noticeably better performance, though I've never checked the numbers. A desktop computer would have such performance as standard thanks to its better cooling, I think.

Gopher seems to get good fps with enband lots of demanding modssuch ad interesting npcs?

What model of graphics card does Gopher have for his PC?

Yeah I meant to say decent settings not indecent way to go voice to text!