To answer the question, Kaplan Premier Program is your basic introduction to the GMAT. It gives a run down on what the GMAT is, how they score, and introductions into the types of questions you will see. It also has a lot of practice questions that will help you get used to what the GMAT is looking for. If you have the CD with the Kaplan Premier Program, you will get practice questions on the CD that will help you practice timing in a computer format, which always helps. The only caveat is that the Kaplan program tries to use tricky wording to make the problems more difficult. Your score on the Kaplan CAT's will be significantly lower than the real GMAT. If you know that going in, you'll be fine.
Kaplan 800 is exactly what it says. It gives you ideas on how to solve hard questions. There are not a lot of practice problems, but they are good problems with pretty good explanations.
I think a combination of the two is a good way to get ready for the GMAT, along with the GMATPrep tests from mba.com. You would want to save the Kaplan 800 for when you are ready to approach more difficult problems, while using the Premier Program will give you a good introduction to the basics.
Note: Kaplan gets a lot of grief on this forum, but you can learn from any material if you know what you're looking to get out of it. Personally, I only used Kaplan Premier and Kaplan 800 with very minimal time on OG11(Problem Solving Only), and I did just fine.