Some of the plugins like SafariStand I think can offer you different searches
There was a program called AcidSearch that essentially let you add a number of search engines to the search tool like in Firefox, but it has never been upgraded to work for Safari 3.x, as far as I know.
And then finally, there's a plugin called Inquisitor... Inquisitor's primary function is search-as-you-type results, but it does also let you specify a different search engine instead of Google.
Inquisitor seems to be the most popular option. I use it and I also use SafariStand, but I use the latter for a different purpose.
If you type in a search, you get the popup inquisitor box, with some search results, and also at the bottom some options to show the search results pages at your specified search engine, etc. When you click on those, you go to the normal Yahoo Search or whatever. Also if you type your search text in and press <enter> like you normally would, then the pop-up disappears and you get the search page from your engine as normal also, so yes.