Forum posting is good for increasing traffic to your site, but has no effect on SEO rankings directly. Increased traffic should translate to more sales.
That's not true. Forum posting in most cases does impact SEO. To Google there's no difference between a link from a blog post, forum or website. Both are outbound links and will pass page rank. Sure the blog will likely be valued higher as forums tend to have a lot of outbound links and duplicate content but if the forum is of good quality then it will impact SEO positively. Especially forums such as webhostingtalk.com where they actively ban spammers and those pasting copy/text martial.
How much benefit you get for SEO is also going to depend on some factors such as which posts you're replying to (quality forum, unique content, high pagerank of the forum) and how you've setup your signature.
For example if you link to your website using:
http://www.yourdomain.com/
Or:
CompanyName
You're missing out on the possible SEO benefits. For the best effect you'll want to link to your website with keywords that you're looking to increase your index rank for.
E.g:
"web hosting florida" links to your web hosting page
I would not recommend linking your company name because very quickly Google will already show you as the top result when someone searches your company name. You're building links using a keyword you're already ranked for.
The only acceptation to this is forums that use rel="nofollow" on outbound links (very few do this) or poor quality forums that Google doesn't value thus ignores or holds less weight to outbound links from the forum.
I've been studying SEO for over 6 years and using what I've learned. I plan on writing a e-book on this very subject.
PS: I'm a big fan of this forum, it's one of my favorites.
