RHCE exams are included with RH300. It can also be purchased on its own.
RHCE exams occur on the fifth day of all RH300 classes
http://www.redhat.com/training/rhce/courses/rhexam.html
RHCE stands apart from many other certification programs in the IT sector because of its emphasis on hands-on, performance-based testing of actual skills in Red Hat Linux installation, configuration, debugging, and setup of key networking services.
See RH300 course prerequisites. For further information, please refer to the RHCE Exam Prep Guide: www.redhat.com/training/rhce/examprep.html