Only a flying visit at the moment will be back on later...
A forum signature link counts as a link to your website and as the SE's (in particular Google) treat links as a measure of the websites relevancy (or authority on the subject) then yes it will help in your SERP's.
The key to linking (apart from quality/quantity) is to have a mixture of anchor text links, ie. different keywords so that it forms a more natural linking strategy because SE's are becoming more and more negative towards unnatural website development. That also applies to the development of content for your site, and postings to your blogs etc etc.
Also a lot of people assume that you always have to have links from higher PR rated web sites, this is not the case as once again this is unnatural and it is always best to have a mixture of new/old sites low/high PR sites etc. Again if you think about this it is more natural in the life cycle of a website that this is true.
Also, try to spread the pages that these links link to. Do not have them all pointing to your homepage, try to spread them so that they point to a number of your pages so once again to establish a natural linking development.
A trick I have always used and Google have just started to formalise (Google Site Maps) is to have a few pointing towards your web sites site map (always include a sitemap) as this lists all your web pages and will provide not only a useful resource to the reader but will also provide a 'hub' of links for the SE spiders to follow when they index your site map and hence index your site..
Example site map :
http://www.onlineauctiontrader.com/sitemap.htm (not a Google Site Map)
I will illustrate this using pages from one of my sites when I am out of the 'Google Juggle' that is happening at the moment, my sites are jumping/dropping PR almost hourly..not unique to me quite a few are experiencing it and hence the flying visit as I want to keep an eye on things and monitor/track for reference etc..
Be back later