Louisville was ranked 11 IIRC and we may have kept them from going to a new years six bowl game. It also took us from a crap bowl to what was the gator bowl, a new years day game.
I would consider it a big win, if not a "signature" one. That's a win against a rival with a (then guaranteed future) heisman QB. After this I have no doubt motorcycle Bobby was pissed beyond belief. I'm kinda irritated that Gary wanted to ignore this game, as it was a big win, which is why I posted about it earlier.
South Carolina is a solid win this year as well, though not a signature one. They're ranked now and may upset Clemson.
As far as overall, he has not beaten Florida or Georgia (and God knows I wish we had beaten Florida and Ole Miss this year) but, he has managed to get a 4 game win streak against South Carolina, 3 against Mizzou, and he's beaten Vanderbilt, Tennesee, and Mississippi State on top of Louisville since he got here. He has been doing a very good job based on what we were when he got here and the fact that we are not known for being a football powerhouse. We could be in much worse shape.
Stoops has definitely made some questionable in game decisions (like not taking out the starters when Georgia pulled away). But we are getting closer to respectability and not being considered an automatic win by every school we play.