In his talk, Dierk König showed ways to test a web applications with canoo webtest. There are several way to come up with a test script: Recording the user actions with a browser plug-in or by writing the script directly, for example.
He also explained the best ways to test an old application (just test what you can, look for NPE's, for example, load all pages, make sure some properties appear). Of course, since the app wasn't designed to be testable, you'll be limited in what you will be able to test.
IFRAMEs, on the other hand, are not a problem, as are AJAX requests. The main issue with AJAX is that while they are still asynchronous, the test framework has some limitations as to what it can test (permutation of request order). Testing special code for IE and FF is not a problem, the framework supports some browser bugs, too. Also, you can have special code to login or to get the app into a certain state and use this code as a kind of "subroutine" in several tests to avoid code duplication.
It's not really suitable for load tests, though, since it lacks the features to run concurrently on several computers at once. Use JMeter for that.
If you need to prepare the DB before the tests, use dbunit.