Will there be any benefit from running Xbox games on the Xbox 360??? 
Yes, ALL Xbox games will be able to run at 720p (1280x720) with full screen anti-aliasing (FSAA) on the Xbox 360.
Which Xbox games will be playable on the Xbox 360 at launch?? 
The following list below is the list of games that will be playable on Xbox 360 at launch:

AirForce Delta Storm Alias (Xbox) Aliens versus Predator®Extinction All-Star Baseball®2003 Amped®Freestyle Snowboarding Army Men®Sarge's War® Atari® Anthology™ ATV: Quad Power Racing 2™ Baldur's Gate™: Dark Alliance™ II Barbarian Barbie™ Horse Adventures™ Wild Horse Rescue™ Batman Begins™ Battle Engine Aquila Battlestar Galactica™ BMX XXX™ Brute Force (Xbox) Buffy the Vampire Slayer™: Chaos Bleeds Cabela's® Dangerous Hunts™ Cabela's® Outdoor Adventures™ 06 Cabela's® Deer Hunt™ 2005 Season Cabela's® Deer Hunt™ 2004 Season Call of Cthulhu®: Dark Corners of the Earth™ Call of Duty™: Finest Hour™ Casino Catwoman™ Chicago Enforcer Circus Maximus Close Combat: First to Fight Codename: Kids Next Door: (Xbox) Colin McRae Rally 4 Combat Elite: WWII Paratroopers™ Commandos 2: Men of Courage™ Conflict: Desert Storm Constantine™ Crash Twinsanity™ Crash Nitro Kart™ Crimson Skies®: High Road to Revenge™ Crouching Tiger, Hidden Dragon (Xbox) Curse: The Eye of Isis (Xbox) Dark Angel Darkwatch™ Dead or Alive® 3 Deathrow Digimon® Rumble Arena® Dinotopia Drake™ Egg Mania™: Eggstreme Madness ESPN MLS ExtraTime 2002 Euro 2004 F1 2001 Fable (Xbox) Fable®: The Lost Chapters™ Fairly Odd Parents: Breakin' da Rules FIFA Soccer 2003 FIFA Soccer 2004 FIFA STREET Fight Night 2004 Fight Night Round 2 Ford Mustang Ford vs. Chevy Forza Motorsport™ Freedom Fighters™ Frogger Beyond™ Futurama™ Fuzion Frenzy® Genma Onimusha Goblin Commander™: Unleash the Horde Grand Theft Auto 3 Grand Theft Auto: Vice City Grand Theft Auto: San Andreas (Xbox) Gravity Games Bike: Street. Vert. Dirt. Grooverider: Slot Car Thunder (Xbox) Half-Life 2 (Xbox) Halo® Halo 2 (Xbox) Harry Potter and the Goblet of Fire™ Harry Potter and the Sorcerer's Stone™ He-Man: Defender of Grayskull Hitman: Contracts™ House of the Dead 3 IHRA Drag Racing Sportsman Edition IHRA Professional Drag Racing 2005™ Jade Empire™ James Bond 007™: NightFire™ Judge Dredd: Dredd vs Death (Xbox) Jurassic Park: Operation Genesis Kabuki™ Warriors Kelly Slater's Pro Surfer (Xbox) kill.switch (Xbox) Lemony Snicket's A Series of Unfortunate Events™ LOONS-The Fight for Fame Manhunt Mat Hoffman's Pro BMX™ 2 Max Payne Max Payne® 2 Medal of Honor European Assault™ Medal of Honor™ Frontline Medal of Honor™ Rising Sun Mega Man® Anniversary Collection Metal Arms™: Glitch in the System MicroMachines Mike Tyson Heavyweight Boxing Monster Garage™ Mortal Kombat®: Deception™ MTV Music Generator 3 (Xbox) Murakumo: Renegade Mech Pursuit MX World Tour™: Featuring Jamie Little Namco Museum™ NBA LIVE 2003 NBA Live 2004 (Xbox) Need For Speed™ Underground 2 NFL Blitz 2002 NFL Blitz 2003 NFL Blitz 2004 NHL® 2004 NHL Hitz 2003 (Xbox) Ninja Gaiden® Ninja Gaiden® Black Outlaw Golf 2 Outlaw Volleyball Pariah™ Phantom Crash™ Pinball Hall of Fame™ Pitfall®: The Lost Expedition™ Predator Concrete Jungle™ Prince of Persia: The Sands of Time™ Pro Evolution Soccer 5 Pro Race Driver™ Pump It Up™: Exceed Pure Pinball Puyo Pop Fever Quantum Redshift (Xbox) Rayman Arena Raze's Hell™ Red Dead Revolver Red Faction II (Xbox) RedCard™ 2003 Robotech: Battlecry Rocky™ Legends Rogue Ops Samurai Jack Samurai Warriors™ Scooby Doo!™ Night of 100 Frights Scrapland (Xbox) SEGA GT™ 2002 Shadow The Hedgehog™ Shamu's Deep Sea Adventures ShellShock: Nam '67™ Sid Meier's Pirates!® Sneakers™ Sniper Elite Soccer Slam Sonic Heroes™ Sonic Mega Collection™ Plus Speed Kings™ Sphinx and the Cursed Mummy™ Splat Magazine Renegade Paintball SpongeBob SquarePants™: Battle for Bikini Bottom SpyHunter 2 (Xbox) Spyro: A Hero's Tail (Xbox) SSX 3 Stake (Xbox) Star Trek: Shattered Universe Star Wars Episode III: Revenge of the Sith (Xbox) Star Wars®: The Clone Wars™ Star Wars® Jedi Knight®: Jedi Academy™ Star Wars: Starfighter™ Special Edition Star Wars®: Knights of the Old Republic™ Star Wars® Knights of the Old Republic® II: The Sith Lords™ Street Racing Syndicate™ Stubbs the Zombie™ in Rebel without a Pulse™ Super Bubble Pop Super Monkey Ball™ Deluxe SX Superstar™ Tecmo Classic Arcade (Xbox) Teenage Mutant Ninja Turtles™ Test Drive®: Eve of Destruction Tetris Worlds™ The Great Escape The Hulk (Xbox) The Incredible Hulk™: Ultimate Destruction The Incredibles: Rise of the Underminer The Lord of the Rings™: The Return of the King™ The Simpsons™ Hit and Run™ The Simpsons™ Road Rage Terminator: Dawn of Fate (Xbox) The Thing™ Thief: Deadly Shadows™ Tom Clancy's Ghost Recon Tom Clancy's Rainbow Six® 3 Tony Hawk's American Wasteland Tony Hawk's Pro Skater 4 (Xbox) Tony Hawk's Underground 2 (Xbox) Tork™: Prehistoric Punk™ Toxic Grind (Xbox) Ty The Tasmanian Tiger™ Ty the Tasmanian Tiger™ 2: Bush Rescue™ Ty the Tasmanian Tiger™ 3: Night of the Quinkan Urban Freestyle Soccer Vexx™ Volvo: Drive for Life World Series® Baseball 2K3 Worms 4 Mayhem Worms™ Forts: Under Siege WWE™ Raw™ 2 XIII (Xbox) Yourself!Fitness™