Hello, I'm here to share with you with my setups which allow you play Skyrim with rather comfortable FPS and pretty graphics.

So, what I have done:

1. I set texture quality on high and anisotropic filtering on 16x, because as for me even today's low-end PC could cope with it;
2. Now speaking about shadows. They were the most problem how to make them look pretty nice and don't load system very much. So I preferred low resolution but far LOD distance shadows rather than mid resolution shadows but with very, I'll repeat, VERY short LOD distance. I've found it quite ugly. Also, I turned on shadows on trees and grass, 'cos I think it isn't decrease FPS too much;
3. I set resolution on 1024x600, because on my 15.6" screen it looks OK. Of course I could have made resolution 800x600 and it wouldn't have lagged at all but it would have looked too ugly (sorry for too many conjunctive moods :)).

And now a mod which I strongly recommend you to use to have a performance boost it interiors (like dungeons, ruins, caves etc.):
http://www.skyrimnexus.com/downloads/file.php?id=736 (thanks user 747823 for this)
If you have weak PC like me it would help you quietly.

Also if you don't like low quality scenery you could use this mod:
http://www.skyrimnexus.com/downloads/file.php?id=1598 (thanks to user OpticShooter)

And my system configuration is:
CPU: AMD Turion II Dual-Core Mobile M500
VGA: ATI Mobility Radeon HD 4200
RAM: 4 Gb
OS: Win 7 64-bit

Wish you a comfortable game!

P.S. I'm not a native English-speaker :)