I don't know of anything that would specifically tie in to Google Talk, but one possible option would be to install voice recognition software and have it listen to the incoming audio and transcribe from there. There are a lot of voice recognition programs out there, but if you are running Windows 7 it has one built in (you can find it under
Start -> All Programs -> Accessories -> Ease of Access). The trick to getting it to listen to the conversation and still allow you to talk into the microphone would be to enable the "Stereo Mix" setting on your computer (or similar workaround). This allows you to set one of your recording devices to be the output of your soundcard. If you are able to set your voice recognition to listen to the soundcard, then it will transcribe what the other party is saying while allowing you to still speak through the microphone.