How important is it to have a LOT of internal pages linking to a particular page or topic. Outside of links and content - is having more pages (in our case, product pages) pointing to a category/keyword helpful in SERP ranking?
It helps a lot IF those internal pages linking to the "particular page" are relevant to the topic of the page being linked to AND if the link text used to link is relevant. I changed our site last year to use pyramid themes (see Brett Tabke's post 4 down on the thread - red/blue diagram) and our rankings skyrocketed. I believe it was in large part because of the use of pyramid themes. With pyramid themes you basically create verticals within your site about different topics. The deeper you drill into the vertical by following links, the more specific the pages get about the topic and the more longer tailed the keyword phrases become. The pages within a vertical try to mainly link up to their parent page (which is about the same topic) or down to their children pages (which are more specific about the same topic). There should be very little cross linking between verticals, sub-verticals on the site (except for your global navigation - can't really do much about that). So pyramid themes not only better control the flow of PR around your site, but also control the flow of relevance. It's an old architecture but works great for ranking at Google where relevance of the pages which are the source of incoming links is so important.
Take a look at Wikipedia and you will find your answer. Why do they dominate the SERPs? Dozens of links on every page to other Wiki articles, each with highly targeted anchor text? Don't confuse run of site menus with contextual linking though.
It is very important because it provides your internal pages with better pagerank values and can therefore be more easily found through search engines. This is why having internal links from your main website page counts a lot.
It's extremely useful, especially if you're using internal contextual links. Click through to the article, and you'll see why internal content doesn't always need to be contextual to benefit you in SEO terms. In general terms, more content also means more chances to benefit from long tail searches.
A proper internal linking and a big site with many internal pages could help quite a bit in terms of SEO. Especially in some less competitive niches you will be able to rank high even without countless backlinks
I run a wiki and it dominates on Google for the particular game... mine isn't even the official wiki for the game, it is the unofficial one and it ranks better than the original. It depends on content and how well you can link things together. Personally I love my wiki and think that websites should be done the same.
This is one of the on-site seo work that we all need to do. Apart from maintaining good meta tags, interlinking related posts and articles with proper keywords does help improve our ranking.