Even if my websites in my sig don't relate to the topics on Digital Point, will my sig backlinks still help with PR and SERP's?
Yes, because DP does not nofollow the links, and many threads acquire PR in time. Not EVERY single backlink has to be on topic to help with SEO, but you want the bulk of them to be. If you have an opportunity to pick up a PR 8 backlink, (on a page with say 30,000 backlinks) you're not gonna turn that link down just because the topic doesn't relate exactly to your site.
If it doesn't relate it will get less credit towards PR. You can get a link joining the forum on my sig as well.
As Benahue said, the links here are not no-followed and will thus pass along PR, but due to the massive (and I do mean massive) amount of external links in a typical forum thread (here and elsewhere), those links won't be worth much. They'll help, but don't depend on them as your primary source of PageRank and traffic.