So I think I've sorted some things out. I may be wrong, but I'm probably right.
The game will be downloadable as Sonic Adventure, with the Dreamcast title screens and all the features found in the Dreamcast original (however the character models will be from DX). The total achievement score of the basic game will be 200G
Sonic Adventure DX will be available as DLC. Downloading DX will change the title screen and menus to the ones found in DX. Additions will include the 60 missions and Metal Sonic (whether he is available from the start or must be unlocked with all emblems is unknown). Downloading DX will allow players to receive an additional 50G.
How do I think I know this? Well, we've seen both DX and Dreamcast menu screens in leaked footage. DLC is confirmed per the Dreamcast-style title screen. The usual XBLA release is 200G achievement score, however SA has 250G. The last three achievements total 50G and are DX specific (Emblems, Metal Sonic, missions).