Ok so I've been playing for a day or so on the hard difficuty setting and its not really all that challenging so I was wondering if I should up it to nightmare...which I prolly woulda done from the get go but this ID we're talkin about....

Mabey I havent progressed far enough for it to get truely challenging ( still on first disk )

Any advice from people that have played all the way thru on either hard or nightmare?

Anybody know how changing the difficulty would effect my end game achievements?