When Microsoft announced that Xbox Live would add a Games on Demand marketplace on August 11, the company revealed a handful of games for the upcoming download hub. Today, Microsoft announced the list of the 21 games for the Xbox 360 game library on Games on Demand for Xbox Live's 20 million-plus users.

Call of Duty 2 will be storming onto Xbox Live in less than a week.

The Games on Demand marketplace will replace the current Xbox Originals store that offered only original Xbox games. Both Gold and Silver Xbox Live subscribers will be able to directly purchase digital copies of the newly revealed upcoming titles for the Xbox 360.

[UPDATE 2] When asked about the games' pricing scheme, a Microsoft rep told GameSpot the following: "We aren’t detailing pricing by title quite yet, but Games on Demand titles will cost roughly the same as you would expect to pay for these titles in a retail store, from $19.99 to $29.99 (USD). You’ll be able to pay with both credit card and Microsoft points, which will now also be available for purchase via PayPal on Xbox.com Web marketplace."

[UPDATE] After initially sending out the UK Games on Demand launch list--which didn't have BioShock or Sonic the Hedgehog--Microsoft has provided the full US list. Both lists are below:

US GAMES ON DEMAND LAUNCH CATALOG
Assassin's Creed (Ubisoft)
BioShock (2K Games)
Burnout Paradise (EA)
Call of Duty 2 (Activision)
Dance Dance Revolution Universe (Konami)
Fight Night Round 3 (EA)
Kameo: Elements of Power (Microsoft Game Studios)
Karaoke Revolution American Idol Encore (Konami)
LEGO Star Wars: The Complete Saga (LucasArts)
Mass Effect (Microsoft Game Studios)
Meet The Robinsons (Disney)
MX vs. ATV Untamed (THQ)
Need for Speed Carbon (EA)
Need for Speed: Most Wanted (EA)
Perfect Dark Zero (Microsoft Game Studios)
Prey (2K Games)
Rainbow Six Vegas (Ubisoft)
Ridge Racer 6 (Namco)
Rockstar Games Presents Table Tennis (Rockstar Games)
Sonic the Hedgehog (SEGA)
Test Drive: Unlimited (Atari)
The Elder Scrolls IV: Oblivion (2K Games/Bethesda Softworks)
Viva Piñata (Microsoft Game Studios)
Viva Piñata 2: Trouble in Paradise (Microsoft Game Studios)

UK GAMES ON DEMAND LAUNCH CATALOG
Assassin's Creed--Ubisoft
Burnout Paradise--EA
Call of Duty 2--Activision
Elder Scrolls IV: Oblivion--2K Games
Fight Night Round 3--EA
Lego Star Wars: The Complete Saga--LucasArts
Kameo: Elements of Power--Microsoft
Mass Effect--Microsoft
Meet The Robinsons--Disney
MX vs. ATV Untamed--THQ
Need for Speed Carbon--EA
Need for Speed: Most Wanted--EA
Perfect Dark Zero--Microsoft Game Studios
Prey--2K Games
Rockstar Games Presents Table Tennis--Rockstar Games
Sega Rally--Sega
Test Drive: Unlimited--Atari
Tom Clancy’s Rainbow Six Vegas--Ubisoft
Tomb Raider Legend--Eidos
Viva Pinata--Microsoft Game Studios
Viva Pinata 2: Trouble in Paradise--Microsoft Game Studios

Microsoft already revealed some avatar apparel through a preview video, like the steampunk-themed set.

The Games on Demand service is only part of Microsoft's comprehensive update of Xbox Live. The overhaul also includes new Netflix features for Gold subscribers and the Avatar Marketplace, which allows players to purchase new clothing and items for their in-game virtual avatars. Xbox Live will also feature a new five-star rating system for any item on Xbox Live Marketplace, the ability to sort friends by Gamertag and status, new options for viewing achievements progress, and improved matchmaking support.

Microsoft also announced today some details behind the avatar apparel. The virtual mannequins can be outfitted with brand-name clothing from Adidas, Quiksilver, Roxy, and Tokidoki, along with game-themed attire from Star Wars The Clone Wars: Republic Heroes, Gears of War 2, Fable II, The Secret of Monkey Island Special Edition, Halo 3, Halo 3: ODST, and BioShock 2.

However, several Xbox Live features promised by Microsoft during the 2009 Electronic Entertainment Expo won't be part of the August 11 update. The company said that 1080p video playback, Zune video downloads, and integration with Twitter, Facebook, and GameSpot sister site Last.fm are all due "later this fall."

For more information on the Xbox Live update, check out the video below.