I mentioned this on JoyMonkeys Wiki page for PowerMAME but thought I would throw it out here too.
If you don't like the default "Microsoft Sam" voice that comes with Windows XP, you can download a speech SDK that includes some more voices. They are better quality and different voices. I use Microsoft Mary and it sounds good.
Download and install the SDK from
http://www.microsoft.com/speech/download/sdk51/You'll notice they have a voice pack too but you will need Dev Studio to integrate the files into the SAPI5 database so it is a big fat hassle. You can get the same voices by just installing the SDK. You can delete the stuff from the SDK that you don't want if disk space is an issue.
Later,
Mike