Apple has an impressive preview of the new voice that will premier in Leopard. However, we don't have to wait for Leopard to get better voice quality than what is offered in Tiger. I have no affiliation with the company I'm going to mention. The Cepstral William voice is the best voice I've ever heard for OS X. I've been using it for several months, and it is the first voice that I actually like enough to use all the time. You'll actually want speech alerts with this voice. It's not free, but if you're into text-to-speech, it's well worth the price.
Apple's Alex voice sounds pretty darn good and I may switch to that with Leopard. But for now, Cepstral William is the way to go.
[kirkmc adds: Note that these voices are not free, but judging from the samples on the site, they do sound pretty good...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2006092316475596