I highly recommend upgrading to the DKC-850 and have done so myself on a DC5 MkIIxg Pro. It not only allows greater flexibility with music formats, but greater storage and access (PC, USB drive(s), Internet, iPad/iPod control, and more).
There are a few downsides: 1. The musical instrument voices are of a lesser quality than the native synthesizer card included in prior models. 2. It does seem to playback and slightly lower output volumes to my external amplifier. 3. It does not play native DK Pro sons/files. 4. If you search this site you'll also find some functions for recording and editing that are lacking in the 850, but most users will never use most of these additional functions for day-to-day playing.
However, these can be overcome by using the "add-on" and not the "replacement" setting on the DKC-850 for most models. Because this can be easily switched back and forth with one setting and one cable swap you can have the best of both worlds. If the old head-unit is broken or you don't want to have both units under the piano, I'd still recommend the 850 as that's the technology direction Yamaha is taking with new Disklaviers and future enhancements.