After you install a Sapi voice, it should appear in the system control panel. If it doesn't, try restarting.
In any case, those synthesized voices are
not going to be much better than Microsoft sam.
If you want a natural sounding voice, you should consider purchasing one of the natural voices from realspeak/nuance, loquendo or AT&T for example. Many of the voices can be purchased for about $30 from nextup.com. These 'natural' voices use speech concatenation to form words and sentences rather than using a tone based
synthesizer. So for example, a person records their voice saying 'ap' and 'el'... When you put in apple, the software puts 'ap' and 'el' together to say 'apel'. This sounds considerably better.