Would it work to run: xsite.com Containing links to other pages on your site with high keyword content instead of running 20 diffrerent sites and paying hosting for all of those sites. Ex: xsite.com--hyperlinks to: xsite.com/highpayingkeyword1.html xsite.com/highpayingkeyword2.html xsite.com/highpayingkeyword3.html xsite.com/highpayingkeyword4.html xsite.com/highpayingkeyword5.html xsite.com/highpayingkeyword6.html xsite.com/highpayingkeyword7.html xsite.com/highpayingkeyword8.html xsite.com/highpayingkeyword9.html xsite.com/highpayingkeyword10.html xsite.com/highpayingkeyword11.html Istead of: xsite.com toxsite.com threexsite.com etc. Does that make sense?
It depends on the context and the use of your site. If you have a bunch of totally random content it just seems out of place, but if you have some kind of commonality to tie it all together it can work. There are two schools of thought here. Some people like to expand on one big site to take advantage of size and PR, while other people like to have a bunch of smaller sites to be more diversified and spread risk. Either way can work.
If those other sites will not link back then choose option one. In that case PR will multiply across the webpages. But if the other sites will have several webpages and also link back to xsite.com I would get them on different domains and different hosting accounts with different C class IP's. Then you will get more value of those links. Correct me if I am wrong.
If they are different keywords all based around a general topic, then yes that makes sense, otherwise no. SE's are moving more and more towards 'theming' websites, so if your site is determined to be about cars and you have a page on that site targeted at football, then that page will be considered less relevant than if the same page were on a football site.
I think the second option (unique domain) will also give better targeting...a domain will get a "theme" from adsense and will carry relevant ads across all new pages right away. But if each page is very different (as in the first option) you may get more off target ads.