I'm currently in the process of downloading "HD" textures for Skyrim via Skyrimnexus.com and I wanted to know how much of a performance hit I should expect, if any?

Here's the link to the specific mod: http://skyrimnexus.com/downloads/file.php?id=607

I'm running with:
i7 @ 3.4GHz
8GB DDR3 1333 MHz RAM
Nvidia 540 GT 2GB VRAM
750GB HDD @ 7200 RPM

Note: I'm running the game at "720p" resolution rather than full 1080p. Also using 8x Anti Aliasing and 2X Antistropic Filtering

I'll report back here on how it affects my performance (it'll take an hour or so, slow download)

I didn't notice much of a hit. Of course, I have other settings lowered and am not on ultra, but the actual textures are a great improvement. I have an older 2.1 Ghz Phenom Triple Core with a Radeon HD5670 and 4G RAM.

I have a 4 gig quad core (2.4) with a GTS 450. Very mid-range system, yet I can run everything on full. Dunno what the frame rate is but I get no visible lag or stutter so I'm happy. Download the 'no stutter' mod (more of a patch actually) from Skyrim Nexus and you'll be amazed at the difference. The game was made to run on mid-range systems - and consoles - so the frame rate problem might just be an optimization glitch.

Also don't forget a driver update. I had the flashing sky the first few days till I figured that one out.

Good to know, thanks guys. It should be done in 45 minutes or so I can't wait till they release the CK so we can fix the major lag areas, especially the particle effect ones

Interesting I used a radeon hd5700 card on a six core 8 gb amd rig and had hourly blue screens then I used any texture mods. Replaced it with and HD 6800 and it work well but has not looked into hd textures except some face updates who caused the first crash.

I am running several HD texture mods (realistic water, 2k textures, realistic body texture, etc)

The textures in themselves don't do a lot as far as lag. What they do, is increase load time and can delay LOD updates. But it really depends on your view settings too. You can have 12k textures is you want with minimal increase of lag if you have all the view distances on minimum.

I have a 2.8 6-Core Phenom II with 14 gb Ram and a 6950 2gb Radeon, and even my framerate drops considerably when I'm in the middle of Markarth in some cases it goes down to 20 FPS and yes, I have everything maxed save for AA.