If you passed IBM Exam 399, then you'll likely pass the WebSphere Portal IBM Certification
Test 829 as well. After all, they both test you on the same JSR168 API. The big difference is knowing the wizard support for C2A (WebSphere's Click-to-Action) wizards in IBM's Rational Application Developer 7. And know some of the other wizards that have changes from IRAD 6.
There's also a question or two on the Ajax Portlet framework, mainly knowing about the XMLResponse object and stuff.
But even if you miss the new stuff, there's enough JSR168 stuff in there that will give you a passing score on exam 829. Knowing the other stuff will really just put you over the 80% mark.
It's a good exam, and a tough test, especially if you're not familiar with the portlet specification JSR-168.
-Cameron McKenzie