htaccess has no effect over what window links open in.
The best bet would be use PHP's output buffering to 'catch' the output, then find out-going links and rewrite them as such to add the extra tag needed.
You can use the JavaScript, which is attached below
It will convert all external links, and add "target=_blank" for each external link, automatically on body "onload".
If you don't have a problem, it can also be done with PHP by making a function
let me know if you want that and i'll have it done for you
but do note you will have to add links using that function like this for example if you want to add a new link pointing to msn.com you would have to do it this way.
Just edit your external links to use target=_blank as an attribute to the a tag. If the links are put there by you, do it manually, if they're being put there by a script, edit the script.