A couple of points – google backlinks and pagerank are seriously messed up and out of date at the moment… if you spend any time on a SEO message boards, it’s pretty evident that this is the case – so in terms of it only showing 1 backlink for you at the moment, I wouldn’t worry too much.
Another thing, when you search for backlinks in google (i.e. link:http://www.domain.com, search backlinks from the toolbar etc.) it only shows the links from pages with a PR of 4+ – so if pages with a lower PR exist to your site, they still count, they just won’t show up when you search for them.
…and Internal links count heaps – it’s debateable whether they count as much as external links, but they certainly count for something – which makes total sense, the larger the site, the more likely someone has put in an effort to get it that way, and the more likely it is to contain good information. On of the biggest tips professional SEO’s give out, is to add a page a content a day to your site… bigger is certainly better (particularly in terms of width of your site).
It’s all defiintely frustrating though – my site I launched two months ago doesn’t show any backlinks yet, despite over 200 results being returned when I do a search on the domain… I’m really hanging out for a major google update to backlinks and PR 