I've got a 2011 MBA with i7, 4GB, 256GB - Windows 7 64-bit Ultimate in boot camp.
Intel JUST released updated graphics drivers in October/November - Make sure to download those as they will make a difference in performance on the mobile i7/i5 chips (their notes).
1. Skyrim - I have my graphics settings as:
Native resolution/full screen - No AA, No Anistropic filtering, No FXAA, Textures "high", Radial Blur "low", Shadows "low", Decal Quality "high", Water reflects: Land, trees, sky.
I have installed quite a few high res mods and the mod that recompiles the main .dll to take advantage of SSE instructions (improves the frame rate by 2-4 FPS in certain situations specifically on my system).
The shadows, from what I've read, are CPU drawn and poor regardless of settings. - there are a few mods attempting to correct this. I think it's a bug because my roommate has a very nice quad core i7 with an Nvidia GTX 560 graphics card and his shadows are set to "ultra" and still look like poop. I just reduced them to low and deal.
My frame rate runs from 16-25 FPS - and it's playable and looks nice - I'm level 45 BTW.
I like the nicer textures but you can dial them down and get higher fps.
Civ 5 - I play the Windows version because it's native and not using a Wineskin wraper (lazy developers use this instead of creating a native version) It's slow but playable on my system - Noticeable when you are waiting for the computer to calculate the next round. The graphics look good and run pretty smoothly otherwise (battle scenes). I've never had it lock up or crash on me.
I have not played Portal 2 or Starcraft - Skyrim pretty much sucks all your free time (you've been warned)
Let me know if you have any questions.