kumarkumar - There's no problem targeting the pages according to commonly used keywords, but, that approach, coupled with one of your questions makes me think that they are intended to be doorway pages.
I posted some comments on the subject of doorway pages a while back that may be helpful here....
When is a door not a door?
When it's ajar. But seriously - about doorway pages...
Every page on a site is potentially a doorway page, and IMO that's OK as long as the pages are well-integrated into the site, enough so that Google will see that they are important pages. This means that they've got to be prominent in your site's linking structure, which also means that you've got to make them useful for users, with substantially unique content (they can't just be cookie cutter pages with keywords dropped in).
The point that Marcia raises is very valid. If you've got 50 new pages integrated into your site, they are potentially going to be diluting PageRank that's going to other pages on the same hierachy level... and that could cause the other pages to fall in ranking.
My tendency would be to pay attention to target-phrases in which users have demonstrated interest, but to add them gradually. I realize that 50 pages isn't very many, but used the way I think you're planning to use them, they could be quite a lot.