I'm playing on an old 3.2GHz dual core with 4GB of RAM and an ageing 8800GTS 512MB. I have been slowly moving the graphics settings up and I've been surprised by the performance I've gotten playing it @ 1920x1080 120Hz with most things on high. Ultra is definitely a no go.
The game is great, much more engaging then fallout or the other elder scrolls games. The AI though is not the best. I shot a mammoth by accident while hunting elk and he chased me up a rock. He then just stood there while I shot him with arrows and electrocuted him. My skill in archery went up 2 levels (I was only level 5 at the time).