Except the GTalk's PC client software doesn't have video capability currently.

Really there is nothing special about GTalk though. It's just Google's branded Jabber/XMPP based service. You could make the same video calls from tablet to tablet over any Jabber/XMPP server really. I've done it with a private Jabber server and it worked well.

If there was PC Jabber software that supported jingle-video (like the NITs use), then it could be done with PCs also.

As for what's *best right now* that sort of depends on what you're after.

Certainly aMSN wins for having the most popular PC client. (MSN messenger) I haven't tried it much, but it does work. The aMSN client is a little heavy on the tablet though, so do expect amazing framerates, etc. It's biggest advantage is that you're using a pretty popular PC client.

Gizmo is rather obscure but it still has presence notfication like MSN which you may want. (ie. user away, online, etc) Video works but quailty isn't the best. I've asked them over and over about the "bandwidth test" it does. Even on massive pipes I get "good" and some low rate selected. Still no answer.

If you are after the best quality video and voice, then SIP is it from what I've seen. It's actually quite good IMO. However most people aren't as familiar and related software like Asterisk, Eyebeam, etc.

I've been using video over SIP mainly though, using Eyebeam on Windows PCs. The only real hitch at this point is that I moved to an n810 recently and am waiting on this to clear up:

https://bugs.maemo.org/show_bug.cgi?id=2792

I'd prefer to find a Jabber/XMPP PC client though... still waiting.