If I buy the SEGA Dreamcast Collection for the 360... ...with specific regard to the port of Sonic Adventure Included... Is this compatible with the DLC available on the marketplace, in order to turn in into Sonic Adventure DX? (or is the DLC only compatible with the XBLA version?) I checked on the wiki a while ago but the info regarding this matter wasn't present. Thanks
No it doesn't. You have to download the SA: DX DLC and yes it should be compatible since it's just an arcade game being installed to the harddrive pretty much. You just need the disc to boot them obviously.
They act like Arcade games, but you can only play them while the disc is in the harddrive unlike if you actually downloaded SA:DX from the Xbox Marketplace.