I can tell you that the Network+ exam and the CCNA exams are quite different in many ways. The one thing you should know going into the CCNA exams is that the questions will not be as straight forward as the ones on the CompTIA Network+.
Namely the Net+'s format, for the most part, asks you direct questions about technologies and whatnot. The CCNA exams will have a few of those, but they also want to make sure you know how to apply that knowledge in scenarios and troubleshooting exercises (including simulation of the Cisco Internetworking Operating System).
The best advice I can give is to baby step it. Consider taking the two test route to achieve your CCNA (640-822 ICND1 and 640-816 ICND2). By starting out taking the ICND1, you get your CCENT certification and you are also able to focus your studying on the concepts for the first exam before moving on to ICND2.
This was first published in June 2008