This Enb is for low-middle pc videocards ( like my - GF650Ti Boost/GTX670).
Perfomance with this preset + Purity- in my Skyrim is practically always -60fps.
(All ultra settings)
Except castle Volkihar ,and Markart -  50fps min.
Req.
Purity.
http://www.nexusmods.com/skyrim/mods/65242/?

Instructions:
Open SkyrimPrefs.ini located in My DocumentMy GamesSkyrim

Set these values in SkyrimPrefs.ini:

bFloatPointRenderTarget=1
bTreesReceiveShadows=1
bDrawLandShadows=1
bShadowsOnGrass=1

Download  ENB  for Skyrim -  v266 .
Copy d3d9.dll and enbhost.exe un paste files where the TESV.exe is.
Unzip and copy all files from - MinimalENB For Purity
into your Skyrim folder .

All credits for enb series to boris vorontsov