As you are doing, you can put a link to your signature and create links to your site(s) this way.

However, these links don't carry too much value, if any at all. Signature links are still better than profile links, though, because they come from pages that do have some content on them.

[DELETED]

Even though the actual SEO value of signature backlinks are limited, you do gain some more eyeballs and hopefully some new visitors. This is worth something too :-)

Forum posting can be a great tool to get visitors to your site. Although it does have little effect on your overall SEO plan.