Initially nightmare is tricky as you don't have many skills available or decent items but as it goes on I don't think the game is too difficult really. I've never used a helmet in it for example. Though I have never tried as unarmed fighter either, once you start getting some of the higher tier skills it shouldn't be too hard except for the odd boss. I found as a ranger it wasn't too challenging after the early game anyway.
I currently have it running on a relatively low-end machine (AMD E450 sub-notebook) and once I turned all the shadows and dynamic lighting stuff off or on low it runs really smoothly even with high quality textures so I'm very happy with that. I think I read other people saying that the shadows can cause some freezing and frame skipping so it's possible that's what happened you but odd if it only happens at the very beginning.
