Depending on what "loud" is to you, a muffler/muffler delete may do it (SLP Loudmouths).
The first mod I did was Magnaflows....too quiet, I tried flow-master...louder, but not loud enough...both were marginally louder. Even after my Xpipe install. The car was "somewhat" louder, but not "turn people's heads as you pass by" loud.
That didn't happen until I cut the cats out. It has an aggresive sound now...very noticeable and loud. I don't like the "tone" that the flows have. Kind of sounds like a chamber in the muffler has come un-welded and is rattling.
Here's a vid:
http://216.180.243.6/Files/Ninos2005Stang.wmv
-N