Sorry in advance...I can't answer your question. I have one of my own.
Have you been able to simulate without using the external RAM? I have a microblaze project that I have been able to get working in NC-Sim. Unfortunately, the simulation has to run a LONG time before the system is even initialized. NC-Sim starts getting bogged down and apparently the memory resources used are huge! I then cannot simulate what I am interested in. Do you have any advice?
For your problem, I would think that you may have to use internal RAM resources that have simulation information instead of an exteranal device. You probably can do it with the external RAM, but I bet it is difficult. If you are trying to do timing sim, I guess the internal RAM would not give you the most realistic sim. However, if you are just trying to sim HW/SW interaction, it should be OK.
I have not found much info on simulating a whole ppc/MB system. There is instruction on how to set it up, but once that is done, the info trickles off. My level of knowledge on NC-Sim is limited!
Good luck!!