There is no pre-req exam. Everything you would need to know to pass the GCIA is covered in the class. This is one of the harder exams.
I've taken 8 GIAC tests and for some reason I still think 503 was the hardest. Maybe I slacked on the prep for this but it wasn't easy. But like the others have said, the course was great, it's one of the few courses that I've taken that I would like to take again.
Note, unless you're incredibly well versed in all the topics, I wouldn't try that one without the course and tons of study.
You need to master the seven different tools the prep course teaches or you won't pass. Not "know what they do", I mean master using them in a practical way, all seven. You are responsible for the content of the entire six day course, it's thousands of pages of content and anything is fair game. If you skip large sections because you think they're irrelevant, you probably won't pass.
For God's sake, DO THE LABS. Do every lab in the workbook, then turn right back around and do them a second time. If you do every last lab, you will probably pass. If not, forget it.
This link has some of the best advice I've seen regarding this exam.
Also, when you take the exam, make sure you go to a testing facility with up to date equipment. I used one that had old ass mechanical style keyboards. Not really a big deal for the multiple choice part, but it slowed down my typing speed for the VM portion of the exam which turned out to be a big deal.
I believe there is also a VM section on the GCIH exam now, so it looks like GIAC is bringing that to a number of their exams. Fun, fun.
Of course you should really be practiced enough with each tool that you can just go to work without a book reference because you definitely won't have enough time to teach yourself how to use the tool in the middle of the exam if you've never covered it. I actually tried that approach because I ran out of prep time and it was a spectacular failure. I did all the labs multiple times for my re-test and it worked out much better.
I found a thorough index is definitely more useful for the main books. One thing I noticed that frustrated me to no end is that a topic can start on one page, go on for two more pages, and then conclude with two or three sentences on the back of the third page. Guess what the question on the exam will be based on? That's right, it's not the main idea of the first three pages, the answer will be those last two or three sentences on the back of the page that you would never know are there if you didn't have your books well indexed.
Something I started doing is writing "Continued ---->" in marker at the bottom of any topic that started on one page and then flipped over to a back page. That way during the exam I would always know there was more to look at on a certain topic no matter how many pages were used to cover it.