I'd say they're probably remaking the game. Using the PS1 graphics would underpower the DS (and Soul Edge is sad to play like crap, anyway) and using the DC ones would be too demanding.
Cell: What's address space gotta do with anything? The Cube is 32 bit, the XBox is 32 bit, yet both are more powerful than the Dreamcast. The PS2 is 32bit, too (some claim it's 128 bit since it has four 32 bit processors and 4x32 is 128, right? wrong.), but it's disputable whether it's better than the DC. The x86 is only now making the jump from 32 bit to 64.