No games require the HDD, except for games that have DLC. If you're interested in Fallout 3 for the 360 and have any interest in getting the expansions, you pretty much need a hard drive. Even the Oblivion GOTY installs the addons to the HDD.
At the moment, it's still more cost effective to buy the console with the HDD.
At the moment, it's still more cost effective to buy the console with the HDD.