I did not have chance to test it thoroughly on N800 yet, and I only placed a few phone calls from Nokia770, so I am not in a position to make an educated comparison. There are so many variables in VoIP chain one might wonder why it works at all. To start with, I have my router set up with NAT, that is enough of an obstacle, and while it is configured to do fixed port forwarding to my Linux PC, it does not allow port forwarding for second device. I guess it works with N800 via UPnP, whatever this means. Sound is very clear both on 770 and N800, better than what my Verizon landline can provide even for local calls. But with 770 it sometimes hard to keep a conversation going because your replies sort of cut off the other party's words. It's more like push-to-talk experience. It is not hard to get used to it (my mom did rather quickly) but still annoying. It is much more smooth with N800, both parties can talk at the same time and hear each other. Occasionally, the sound may break for a fraction of second, but it is on par with my experience with Gizmo on my PC (and with Skype too, to that matter). Bearing in mind that most of my calls were long distance placed during holiday season with destinations in Moscow, Russia (pretty far from Boston, MA where I reside), I would rate Gizmo on 770 as B, while on N800 it would get firm A or A+.

HTH,
Ivan