A signature win is much more than an upset. The 2002 game against OU - upset, not signature win. Signature wins don't happen in years where your team goes 6-6 unless they went 1-11 the year before. A signature win is a platform that propels a program to the next level (see the 2006 A&M basketball win over tu). Since Fran, we have only had a couple of chances at signature wins, most notably the Gameday game against OU. We win that and Aggie Football is at a new level. We eventual go 9-3 that year, but that win would have put us into the Big XII title game.
So, if we start the year of well, say 7-1, OU would be a signature win assuming we don't go 0-3 after it. For that matter if we start 7-1 and beat either OU, Neb, or tu, and don't lose to baylor, any of those could be seen as signature wins as long as we don't get blown out of the next couple of games and we go on and win a bowl game.
To summarize, good year + high profile win + no disasters after = signature win