Included with SADX Mod Installer (varies by mod presets):

Sonic Community Hacking Guide for Sonic Adventure DX PC on Sonic Retro.

GameBanana - Hosts a large number of user-created mods for Sonic Adventure DX.

SADX Mod Loader, included with SADX Mod Installer, eases installation of mods without replacing files, as well as configuring game settings and mod options (if available). SADX Mod Installer can also downgrade the later re-releases of the game to the 2004 port. PkR, the author of The Ultimate Guide to SADX Sins blog, also has developed SADX Mod Installer, a tool that eases the installation of MainMemory's SADX Mod Loader and bundles various community-created mods to restore elements from the original Dreamcast version, or enhance the features of the GameCube version in this port. The 2004 PC version has an active modding scene with many modders not only fixing the PC version's issues, but also have worked on restoring features that were altered or removed from the original Dreamcast version of Sonic Adventure.

The 2004 PC version did not receive any official patches, however, it did serve as the basis of the 2010 Xbox 360 and PlayStation 3 ports (with the DX version's extra content separated as paid DLC), the former of which was then used as the basis of the 2011 Steam version (with the DX version's content mostly in-tact) and was later updated in 2014. The 2004 PC version of Sonic Adventure DX, however, is a subpar port that is marred with inferior visual and sound quality compared to previous versions, a broken windowed mode, lacks keyboard remapping, and a camera spinning bug with certain controllers, among other problems see TCRF for details. The game was later re-released in 2003 for the Nintendo GameCube as Sonic Adventure DX as an updated version of the game, with a number of unlockable Game Gear games as an added bonus, however, it also features a multitude of changes that differ from the original Dreamcast version of Sonic Adventure see this blog or this video for details about the changes between the original Dreamcast version and its later ports. The game also featured six playable characters, each with their own unique gameplay styles and perspectives of the game's story. The original Sonic Adventure, released in Japan on Decemand localized for North America, Europe, and Australia between September–December of 1999, was notable for not only bringing the Sonic the Hedgehog series in 3D, it brought a larger emphasis on story elements, featuring full voice-acting with cutscenes animated in-engine and through FMV sequences. It was the first mainline Sonic the Hedgehog game that brought the series into 3D. Sonic Adventure DX is a cartoon third-person platforming game developed by Sonic Team and published by Sega and Activision Value, and was released for PC on September 14, 2004.

Sonic adventure dx pc box art