Yes, it is quite a long and twisted thread, but I feel like it needed to be. Trying to trouble shoot this problem across two forums would have been huge, and its been a massive help finding people who are fluent in both areas I'm dealing with, so thanks again!
@whale-av I still don't understand why mac os would be ok with one router, and not another. That feels weird to me.
I got the WRT54GL v1.1. I've tried re-booting the mac with linksys as my preferred network, but its doesn't make a difference. I still get 'Network is unreachable (51) coming up.
I'll send you my email, very happy for the network help if its on offer! Its just not behaving, or rather misbehaving, in a way that makes sense to me right now.