How to make the "Search with Google" menu option open up in a new tab by default in Safari?
In this article Jonathan Rentzsch creates a new service because he wants "Search with Google" to work in Chrome (his default browser) but a side benefit of following his tutorial is that your search should open in a new tab.
If your default browser is Safari it should open your new tab there.
http://rentzsch.tumblr.com/post/5794765921/search-with-google-using-chrome
Warning: This is essentially a follow directions, cut-and-paste affair but it does involve seeing code. I'm unaware of a easy to install extension to accomplish this same task.