I've noticed a moderate increase in cell loading stutters with the official hi-res textures, it's to be expected I suppose but according Skyrim Performance Monitor my PC is easily coping with it. It all sits in memory with RAM to spare and my CPU, GPU and SSD rarely go over 60%, is this just an internal limitation of the engine?

Win 7 x64

i7 950@4ghz

6GB RAM

GTX 580

Vertex 2 SSD

The HD Graphic pack is not optimized so some of the elements are bigger than they need to be. I think there is a mod that optimized the textures to make them less of a load on the system. There is even an Unofficial Patch for the HD Pack.

Depends. How many mods are you playing?

As for Windows, did you change the virtual memory? Incresing it might help. I also have a line in my Skyrim.ini that allows the game to use a hard drive as a cache and others that are supposed to make the thing lighter to run. Maybe you could try them?

[General]uExterior Cell Buffer=36iPreloadSizeLimit=26214400bUseHardDriveCache=1fMasterFilePreLoadMB=32.0000iNumHWThreads=4uInterior Cell Buffer=3

I've been playing with these lines for ages; I don't even remember where I found them. We would need to find the original values of these settings in order to compare.

I've found that lighting mods seem to make the biggest hit on my CPU. Also, if you've already downloaded the official Skyrim HD texture pack, I wouldn't bother downloading other HD packs. I think the best way to test which mod is giving you trouble is to use the Nexus Mod Manager. It will let you disable mods without deleting them.