The Google Talk application is now available on Google for Windows 2000 and Windows XP. It features voice and text messaging and loads all of your contacts from Gmail. The application is only 900 KB in size.
Google Talk supports voice codecs PCMA, PCMU, G.723, iLBC, ISAC, IPCMWB, EG711U, and EG711A. They are evaluating the Speex codec. They currently use XMPP-based signaling but plan to support SIP signaling in the near future. Google is currently working with EathLink’s Vling product as well as Sipphone’s Gizmo Project for integration with Google Talk.
Google also has a page listing Jabber/XMPP clients compatible with Google Talk.