(First - I want to acknowledge that I did not research this information. Credit goes to a poster named Sloppy on another board, who frequently makes well-researched posts. So this is info I got from him.)
Arkansas is 29-8 all time in the NCAA Tournament in games seeded higher than its opponent (78.4% winning percentage), including 13-0 when seeded at least 6 spots higher.
Going by seeding, the biggest upset Arkansas has suffered is as a 2 against 7 Virginia in the round of 32 in 1984. Future Dallas Mavs coach Rick Carlisle put back a missed shot with 4 seconds left in overtime to give UVA a 53-51 win.