I'm sure you'll be able to turn graphics down.
The main slowdowns will be shadows and special lighting/bloom effects. The fog isnt a big deal--it helps sometimes hide closer distances (like in WOW draw distance).
BTW, that was one of the more tame screens I've seen. I saw some screenshots that were amazing. huge hills, and.. just amazing stuff.. Check IGN's NWN2 area screens!
http://nwvault.ign.com/View.php?view...es.Detail&id=1
Here is a Cave underground:
http://pcmedia.ign.com/pc/image/arti...6064556904.jpg