if you site link is not cached so no value in forum links put other link in forums so you can get good result fast
There is no value in ANY link that has not be discovered by the search engines. Although, that doesn't mean that it won't get discovered later in the future. For this reason, that link may still have value. It just won't be counted until it gets discovered. If you have links that have not been indexed yet, it's best to take action to get them indexed so that they can be counted, rather than just sitting back and waiting for it to happen by itself.
In case G do not crawl that forum page then you link will also not be crawled. This is how your website will not gain anything with this.
Let me share some of the dofollow forums: www.v7n.com www.webicy.com www.digitalpoint.com www.webcosmoforums.com forums.seochat.com
No specific KW is used for that. Here is a list of Do follow forums - http://getinternetmarketingstrategies.com/2008/11/do-follow-forums/ U can go through it.
i think used blog commenting is more effective to help increase traffic and PR for you're website ... and better you must choice do follow blog to give comment ... Thanks for sharing