OK, I know I'm not exactly sitting on a Ferrari here, but a Phenom IIx4 955 Black Edition, 4 gigs of Mushkin RAM, and an X-FI 6850 1GB card(modest overclock) shouldn't be svcking this much. The basic game - no mods, all options left at the default chosen by the launcher for this card(High) - runs no faster than 35 FPS, and can dip down as low as 15. Adding the low-res enhanced texture pack mods(modified Bethesda pack, all-1024 Combined Texture Pack) brings the maximum down to 29-30 FPS, and actually results in some stuttering(as in, short drops to 0 FPS). Adding even the most basic ENB enhancement makes it completely unplayable.

Those FPS counts are taken on the outskirts of Riverwood by the way.

I have the latest drivers(I already tried rolling back, made no difference), my settings are modest, I don't have acres of performance-killing mods, and I've seen people with similar specs claiming to get 40 steady FPS running full 2048-size texture packs AND one of the more intensive ENB presets plus Shadows set to Ultra and loads of SSAO/MSAA/FXAA; so why is my rig getting such terrible results? What really confuses me is that the hardware isn't even struggling; I ran around the outside of Riverwood for twenty minutes with GPU-Z and a few other utilities running; no sign of overheating, no sign of the card running out of gfx memory, CPU never seemed stressed - even my hard drive wasn't thrashing.

Am I missing something obvious here?

I don't think you're missing anything obvious unless it's the resolution you're playing at. I play at 1080p and I have similar cpu + 7850 (+8 gigs) and I'm constantly at 60 fps. My stuttering disappeared when I added 4 gigs of ram (used to have only 4 for months). I used to have to close all other apps when playing Skyrim and it still stuttered. Do you leave other apps like internet browser or chat at the background while playing? Have you disabled Steam community in game? If you're running anti-virus or firewall those two are very likely culprits. For me, now with 8 Gb of ram the stuttering is gone and I can leave other apps open too, suggesting it was probably caused by paging. Also some say 64-bit OS can help to remove stuttering, don't remember why though. 6 Gb of ram is enough for Skyrim, but I just couldn't help futureproofing myself a bit

Also have you tried deleting skyrim.ini and SkyrimPrefs.ini and letting Skyrim create them for you? (don't edit them again before testing!)
What you should definately do is have Steam verify the integrity of your game cache if you haven't done that yet. Maybe you got corrupted files? Or if you have had many mods installed and removed over time you might want to delete your entire Skyrim folder and reinstall Skyrim from scratch (testing performance without introducing any mods at first). That should be the last resort, however, if you don't have the disc or abundant bandwidth. In Skyrim, when not using mod manager, some assets may get left behind by mods distributed as loose files and they can cause all kinds of trouble (scripts are the most notorious).

What version of Skyrim are you running? If it's months old or never updated, I would suggest updating it. And check when those mods you're using were released and if they are compatible with the version of Skyrim you're running. Also I would advice against running any mods that were released before CK came out.

When testing performance use something like FRAPS to get reliable information on fps while ingame, GPU-Z is your friend too as you already know. And finally, simply disabling mods isn't enough in Skyrim. Mods that have loose files/scripts will still load those assets as long as they exist in the skyrim/Data folder. So perhaps all your issues are coming from mods that you think you're not running. Or it may be just your save, if it's seen the addition and subsequent removal of multiple mods? Saves are great garbage collectors in Skyrim so if you haven't tried it yet, start a brand new game without any mods and letting Skyrim recreate the inis.

Also autosave feature can sometimes cause dips in framefrate. Are you using it?

Chances are you installed a bad driver.Find the installer for your graphics driver and use the full uninstall it has.if it is AMD of course it ask do you wish to uninstall.You click yes then click everything on the list.Now if there are extra drivers which are loading in it's place they can cause problems.

There is another program that can erase the old ones.

See riverSweeper

This removes alll the drivers.That load without your permissions.

Clean your system out of only the graphic driver.Then restart

Then install an older version of your graphics driver.(Recommended) Install Catalyst 11.12 or 12.3 Nothing higher.There are issues with 12.2 and up causing your CPU fan to speed up and disables your Cool & Quiet.Causing your room to heat up and your fan noise to get extremely loud!

Background apps are the main reason for performance issues on a machine that can otherwise run the game.

Well, techincally the main reason is the game's inability to use more than 4GB of RAM or 1 core of the CPU. But yes after that, background apps maybe be causing some issues.

The game very much uses more than a single core of the CPU. And the RAM has nothing to do with it. The game rarely reaches the game's 4 GB limit in normal play. As long as the user's system has enough RAM as a cushion for whatever they're doing and a 64-bit OS, the RAM doesn't have much to do with it.