Alright well, your correct in assuming you will need to use the html "_blank" reference to tell it to open in a new window. Its pretty straightforward in where to place it; just find the code for the chat button and find the link tags (the <a href=""></a> stuff) and place the target code in there. Heres and example for the "Home" button in Musiconica to open in a new window:
Find the home button code:// Show the [home] button.
echo '
<td class="mainnav"><a href="', $scripturl, '">' , $txt[103] , '</a></td>';
And then add target="_blank" to the anchor tag. I usually prefer to have the target attribute directly AFTER the url, but it doesnt really matter where you put it. After you add the code, it should look like this:// Show the [home] button.
echo '
<td class="mainnav"><a href="', $scripturl, '" target="_blank">' , $txt[103] , '</a></td>';
Good Luck! =D