What exactly do you mean by "web 2.0 sites"?
Sorry, but that is not going to help you. Any forum worth its salt is going to mark external links as nofollow, just as this forum does, and they will be useless as backlinks. Moreover, dropping links in forum posts is probably a great way to get yourself banned for spamming.
Any links which you can easily obtain for yourself will be regarded as worthless, or nearly so, by the search engines. You need high quality content on a well-constructed site to gain links from other high-quality sites who will choose to link to you. Link exchanges and other such schemes are unlikely to benefit your site. See Google's guidelines on these.
Check my link above - buying links which pass PR is a violation of Google's Webmaster Guidelines.