http://www.freekindlebooks.org has several thousand of the more popular Project Gutenberg texts in MOBI format, and the start to a "mirror" of the Project Gutenberg texts in MOBI format at:
http://www.freekindlebooks.org/MobiM...obimirror.html
You can also quickly and easily convert Project Gutenberg texts yourself by getting the EPUB format and running Calibre's "any2mobi" command line tool on the EPUB file.
Any of these approaches is almost certainly going to be a more pleasant experience than trying to read a .txt file on your Kindle!