Let me put this as clearly as possible. A domain that forwards to an existing site does not provide any Page Ranking benefits to the forwarded domain. In fact the domain name that forwards to the existing website will not even be available in Search Engine Result Pages (SERPs). This is because when you do a forward you are informing Google, Yahoo, Bing, and other Search Engines that this site does not have any content but instead the site exists at the forwarded domain and that should be indexed. The only time when a fowarding domain assists in the Page Ranking of the forwarded domain is when you migrate a website to a new main domain as the Page Ranking will be associated with the new domain instead of starting off as a new website.
If you are looking to increase your Page Ranking within Search Engines the main factor you should focus on is writing unique and worthwhile content. Don’t get sucked into the whole keyword feeding frenzy. Yes you should include terms that are relevant to your content but only if they make sense in the context of your content. If you do buy multiple domain names do so only to secure those domains with the purpose of stopping competitors from buying and using them.