Thanks for the hints everyone!
@Elitewolf: I didn't think of the "new cpu = new rig" formula windows uses. So you're right, I'm gonna go upgrade the CPU first. Which also means having to buy a new board...
@Irishron: You're so right... funny thing is, we don't even have a common account, but I'm going to start a new job next month and we're gonna move there, and she'll expect me to pay my share of the mess. Rightly so, I might add. But it won't hurt anyone if I upgraded my budget to, say, 700 € - that would allow for a board, a cpu and a gpu, right (gpu ~350, cpu ~250, board ~100)?
@Totalheadache: I know that there's no synergy effects, it's just that I've always had ATI cards and AMD processors. That's why I brought up the comparison of Mercedes to BMW - none is clearly better, it's just a question of personal preference.