If the links are legit and connect to good sources I wouldn't nofollow them. Personally, I only use nofollow for paid ads (if they don't run via an adserver) and user generated content.
If you have those 25 links on a single page, it might be a bit too much - unless it's a link page or such. If the are spread on your entire domain, it's no problem.
Don't worry too much about the page rank and think about content and topic relation. If the sites you link to are topically closely related to your own content, then it can actually help your own rankings if they are of high quality. For instance, if your site is about rocket science, it makes good sense to link to nasa.gov (it would actually look strange if there is no link to this rocket-related authority). Not that I want to sound like matt cutts, but if these links add value to your users, then they deserve to be followed:)