It all depends on your tastes, I suppose. I have a personal philosophy of only playing games if I can run them on max settings, or turning off things that I didn't even notice were there. So, in your place, I would wait (in fact, I can run Oblivion almost maxed, but still put it in my waiting list).
My huge backlog helps a lot with that. I have so many games than I can max out that I don't mind waiting for those I can't. I also suppose Nehrim should be a little heavier than vanilla Oblivion, can anyone confirm that?
Of course, in the end, it's your preference.