It doesn't work that way (=with a pc google talk client), you have to install a specific Nokia-made PC program to be able to video talk between a PC and an N800. In the N800 video app there's an option somewhere in the menues to send an invitation. You can select a google talk contact there (the way I got that contact into the N800 was to go to my gmail account and select Contacts, and set that contact to 'Always' for 'Show in Quick Contacts'. For some reason this will get the N800 to import that contact, as soon as you have configured your own gmail identifier on the N800.)
This will send a link to the other party through a google talk chat, if the other party clicks that link on the PC it'll download and install a program on the PC. The program works, but it's not good: For some it works fine (sound, video, looks ok), for others it's either sound only and no video on the N800 but video on the PC (they can see you, you can't see them), or 99% CPU usage on the PC, or both. Plus and/or choppy sound too. All depending. The PC program is not mature.