Here is the honest reality: Sega no longer sells the standalone 2004 PC executable. When they released the Steam version in 2011, the old CD-ROM release was discontinued.

Over the years, Sonic Adventure has suffered from a history of questionable porting. The GameCube version had framerate issues and washed-out textures. The 2004 PC version had similar issues but was technically "open" enough for the modding community to fix.

The "DX" version was designed to refine the 1998 original for more powerful hardware: Enhanced Graphics

The modding community surrounding the PC version of SADX is legendary. They have essentially reverse-engineered the game to improve it significantly. If you manage to acquire the 2004 PC files, you can use tools to:

Originally, Sonic Adventure launched on the Sega Dreamcast in 1998 (1999 in the West). It was a revolutionary title—the first 3D open-world platformer featuring Sonic the Hedgehog, Tails, Knuckles, Amy Rose, and Big the Cat. In 2003, a GameCube enhanced port called Sonic Adventure DX: Director’s Cut arrived, adding Mission Mode, 60 gameplay improvements, and upgraded textures.

For everyone else: