There are a total of 27 Dreamcast games that have been rereleased on Xbox 360:
Bangai-O HD
Crazy Taxi
Daytona USA
Garou: Mark of the Wolves
Ikaruga
Jet Set Radio
JoJo's Bizarre Adventure HD
The King of Fighters '98: Ultimate Match
The King of Fighters 2002 Unlimited Match
Marvel vs. Capcom 2: New Age of Heroes
Marvel vs. Capcom Origins (Marvel Super Heroes + Marvel vs. Capcom)
Radirgy Noa
Resident Evil: Code Veronica X
Rez HD
Sega Bass Fishing
Shooting Love, 200X (Trizeal Remix)
Sonic Adventure
Sonic Adventure 2
Soul Calibur
Space Channel 5 Part 2
Street Fighter III: 3rd Strike Online Edition
Super Puzzle Fighter II Turbo HD Remix
Super Street Fighter II Turbo HD Remix
Triggerheart Exelica
Under Defeat HD
Vigilante 8 Arcade
Virtual-On Oratorio Tangram Ver.5.66
There are a total of 9 Dreamcast games on Original Xbox that are backwards compatible on Xbox 360:
Dead or Alive Ultimate (Dead or Alive 2)
The King of Fighters 2002
The King of Fighters 2003
Marvel vs. Capcom 2: New Age of Heroes
Namco Museum
Puyo Pop Fever
Shenmue II
Street Fighter Anniversary Collection (Hyper Street Fighter II + Street Fighter III: 3rd Strike)
Tony Hawk's Pro Skater 2x (Rerelease of Pro Skater 1 + 2)