One of the things I hear from computer science teachers from time to time is that games turn girls off and so you can't use them for projects. What I have been hearing from people with ties to the computer caming industry though is that girls, or at least women, do play computer games and that they play them in large numbers. Today I ran into a blog post by Dr. Jill Walker that quotes some actual research on the subject.
The short answer is that more women then men over age 23 play computer games. I'm not sure why the numbers for younger girls are not higher though I suspect it is because younger girls have more oppertunities to socialize away from games than women do. If you look at the games that women play what you see are what is called casual games. Think card games, thinking or puzzle games like Mastermind, spelling games and other games that involve more in the way to thinking than good eye-hand coordination. There seem to be a lot of women playing online games like World of Warcraft that involve interactions (other than killing) with other players. That is something to think about.
My experience is that girls did enjoy writing and playing computer games. Its all a matter of thinking about the right games and avoiding the first person shooter.
BTW there is a book of projects for Visual Basic .NET (with a number of casual games included) at MainFunction.com. This link should get you right there. (You may have to create and account or sign in with one to access it. You wouldn't want students to get to the coded solutions too easily.