Search engines are smart :-) i doubt they count the links much at all right now or have been counting all your posts on a forum the same as one for a while. It is easy for an engine to tell if your site is only linked from a forum. It might help you a little more if your site is very new. It could quicken the rate at which a spider gets to your site. If the forum is crawled and finds your link it might get to your site (and hopefully you also have a site map) and then crawl your new site sooner then if you just submitted it directly to Google. However, there are plenty of ways to get your site indexed. If you are just looking for an SEO benefit i would think (and i have no real research to back this up) you would be much better with article submission on content relevant to your site/products/services/ and a link back to your site in the article or "about the author" section rather then forum posts. Another important reason for this is that the article would then get feed out through a larger number of sites and feeds compared to your forum post link.