1) Hit the following link
2) Customize the height and the width according to your need/mood
3) Copy the script that it generates.
4) Paste the script in your sidebar.php just above the last sidecontent div tag ( I use BlixKreig theme)
For Blogger.com
Just create a new HTML/Javascript Widget and paste the script in the content box.
(Google talk Gadget)
ogletalk.xml&synd=open&w=320&h=447&title=Google+Talk&border=%23ffffff%7C3px%2C 1px+solid+%23999999