I have a 2 gig and I'm pretty sure it's the max. I'm sure you've found Internet TabletTalk,
http://www.internettablettalk.com/. It's so comprehensive I found it almost overwhelming.
TechUsers I always find useful:
http://www.techusers.org/category/nokia-n770/.
I like my 770 although I only use it to read ebooks...my first experience w/Linux and I am hesitant to try much w/it for fear of screwing it up, i.e., the infamous "White Screen of Death".