I am taking about getting a sitewide link that has tens of thousands of pages. This will result in all those backlinks with PR 3 and 4. Is this a bad idea?
Certain actions such as buying or selling links to increase a site's PageRank value or cloaking - writing text in such a way that it can be seen by search engines but not by users - can result in penalization. Please review our quality guidelines at http://www.google.com/webmasters/guidelines.html for more information. If you identify problems with your website and make the changes necessary to comply with these guidelines, please do not hesitate to contact us. I would stay away
sitewides are sooo 2004. I wouldn't do them too much anymore. Maybe one on a good related site, but unrelated? Never.
I agree you would get better success with internal linking your pages this site is an excellent example of that http://www.mikes-marketing-tools.com/ do a search for some of his link text to see for your self
Stay away from sitewide links IMO, unless, the link is in such a prominent position that it will create a huge influx of traffic for you at a reasonable price.