What Bilby is trying to say is.. don't learn DB2.
Heh, I took a DB2 course back in College, and got an A. I didn't find it particularly difficult, but I never put it to use either.
Oracle seems to be the way to go, all of your big IT companies will likely be using it. From a user perspective, like simply writing queries, there does not seem to be a big difference between Oracle, MS SQL, and MySQL. I have used all three in back to back to back gigs, and there was very little difference in writing queries. You mostly just have to get used to how each one treats Joins.
At my current job we have a much smaller IT shop than I was used to, and I had to learn Postgres. I hated Postgres at first, the interface is horrendous, but there is still not much difference when it comes to writing queries, so it didn't take long for me to get used to it.
Getting back to the OP, though, I would learn Oracle, as I think the higher paying corporate jobs are going to be found in Oracle. If you are interested in working for a smaller company, or a startup, MySQL, or Postgres are probably more relevant skillsets. You also usually won't have to worry about experience as much, as a startup is more willing to take risks when hiring. Demonstrating your knowledge, and being personable in an interview will help you out more.
I also have a few questions for Underseer. What has your focus in IT been up to this point? Are you willing to make a move from Chicago? The market for developers down here in St. Louis is currently wide open, and very competitive. Our company has been struggling with finding good Java developers, and even more trouble finding full stack web devs (we use Java and Backbone).
Java and Backbone.. exactly what I've been using for the last year and a bit.
I can see how it'd be a struggle to find full-stack. In my town, which apparently is similar in size to St. Louis, almost everyone specialises. You're either a front-end dev, a java dev, a php dev, and occasionally a .NET dev.
The program I graduated out of pumps out some pretty solid, rounded programmers, but there's only about 10-15 really good programmers out of it a year, and the majority of them migrate out of the city eventually.