jwatte:To get good robotic voices, you typically want to run it through a vocoder filter. (Ultra modern people will use more advanced techniques like formant shifting, but that's not simple or cheap right now AFAIK). I don't think Audacity comes with one built-in, but I know that CuBase does. Perhaps CuBase has a demo version you can download and make the voice in? Or perhaps there's another commercial program with a vocoder that has a demo you can use? (Actually, there are USB audio interfaces in the $100 range that come with "lite" versions of software that might be good enough)
As someone else said, the Microsoft Speech SDK might also be an option, just make sure you check the license.
Finally, you can save the file as a WAV file and an LPC codec with a very high compression ratio. The Xbox Live codec might be useful, if you could extract it -- it's way too robotic for my tastes :-)
Speaking of licensing, you might not want to go around recommending that people use a demo to produce content that they will be exploiting commercially. I don't think I've ever seen a software demo licensing agreement that didn't prohibit such things and it's more than a little disturbing how often that advise is thrown around on these forums. More than a little bizarre in this case since you mention the licensing for MS Speech SDK in the following paragraph.