When we tested our child before kindergarten, we chose to do only the IQ test for financial reasons. As it turned out, the school pyschologist was quite willing to do the achievement testing (for free -public school) after they saw the IQ results. I guess if you had all kinds of money, it would be nice to get a baseline achievement level. However, at this age some GT kids tend to start absorbing things at a very rapid pace and any results you get now may seem light years behind what your kiddo may be doing when school rolls around.
As for what to tell your kiddo, there's a nice article by Aimee Yermish on Hoagie's:
How to Prepare Your Child for Testing We basically told our then-4-year-old that he would be going to take some tests to see how his brain works, and that will help us and the school know how to teach him better. He thought the tests were very fun.
Since you're going to a university for testing, do you happen to know if they have much experience testing GT kids? Most psychologists testing kids for educational reasons do not have a lot of experience with GT kids, just a warning.