I've been meaning to write about this for a while.
Whatever you do, do not purchase Cepstral Voice Starter pack from Digium's online store. For starters, I found out after purchase I might add, that at the time (a few months ago) and currently (from the periodic support ticket reports I keep getting) that their proprietary binary blob text-to-speech module (which they force you to use) doesn't work with Asterisk 1.6. (There was no mention of the fact, or versioning info on their storefront).