Is "internal linking" something that Google give value to, and helps increase SERP positions?
Yes! Absolutely. If you have a product that you are trying to promote to the top of the SERPs, then don't JUST link to it from the bottom of the page where the other related products are at.
Link to it with a text link from within the first paragraph of text on that page, too.
For example, if Widget X is the widget you are trying hardest to promote (because your revenue is so good from it), you might say something like:
These widgets are our finest and they are the best-selling widgets after [link]Widget X[/link], which has been our number one widget for the last ten years. Blah, blah, blah...
I haven't been doing SEO a really long time, but long enough to know and appreciate that you are best on trying to promote a couple of keywords instead of trying to get all of them to rank. You should focus on the ones that are most important and try to get those to rank by good internal linking and by getting external links either to those pages, or to your home page with some anchor text that mentions the keywords for which you are trying to rank.
Also, look for which pages on your site have the most inbound links and if they don't already have a link to your most important product / category page, then try and ad one (without it appearing too spammy).
For example: I have some pages on my site that are article pages. They get lots of traffic and act as link bait; I don't sell anything on them, just provide good information. I can get people to link to those pages more easily than I can get them to link to my product / category pages.
So I have links from those pages to my most important product and category pages and I use that for funneling link juice to them.