Keep in mind that you generally don't need much power for a talkbox. The LM386 should be fine. Most talkboxes use high efficiency horn drivers which have SPL ratings of over 100dB @ 1W/1M.. Keep in mind that it doesn't need to be THAT loud.. you're feeding the signal directly back into a microphone and out to your PA
Something like this would do the trick probably: http://www.parts-express.com/pe/showdetl.cfm?Partnumber=292-2502
good frequency response, low price, 102dB 1W/1M sensitivity...
An ideal solution would have a higher input impedance (this is easily achieved by just adding a JFET buffer on the input) but even without that the circuit should work fine. Think about what you're doing here. You're bypassing your normal guitar signal route to a small amp that's driving a horn driver coupled to a tube stuck in your mouth... are you really THAT concerned with the preservation of a pristine guitar signal? You can barely tell it's a guitar anymore! It might as well be a variable frequency buzzer stuck on the end of a tube... A little loss of high frequency content isn't going to kill your sound. And when the talkbox is bypassed who cares what its input impedance is
With all that said, add a jfet buffer to the circuit, they're so easy you might as well do it... better yet, just build a ruby amp and use it to power your horn driver. If you need more power you can always bump up to a bigger amp and repurpose the ruby as a practice amp