Since I have an R31 I am a member of
www.r31skylineclub.com , lots of info and help for fixing them when their not going right! cruises and for sale/wanted sections aswell. I bought my car from this!
There is also
www.rbforums.com for anything with a Nissan RB motor (mine has a RB30E) it isnt that big yet, but it could grow. and
www.svd.asn.au for SVD owners (SVD was Nissan's high performance divison in Oz) and doesnt have all that many SVD owners...but a load of tools.