I have just bought a Dell Dimension 5000 with an NEC 3530 drive. I also fitted an LG 4163. I noticed the same 'issues' with Nero on both drive, but I did not have the problem in an earlier Athlon 64 based PC.
The fluctuating buffer did not seem to cause a problem with video play back on a domestic recorder, but I still did not like to see it.
I have now got rid of the problem. It went away when I stopped using Virtual memory, i.e. setting all the Pagefile sizes to 0. I have plenty of RAM, and this has not caused me any problems (yet) even though I can still run > 15 different apps at the same time. (I do have 2GB fittetd, but I still have greater than 1GB ram freew, even with many progs running.
I do not have a drive that will let me carry out quality scans, so I cannt say what the real difference is, but the Nero Buffer is now rock steady.