The DGX-670 is more or less a totally different beast than the DGX-660 in terms of its features. I've had a look at the Owner's Manual, and I'm reasonably certain you can save an essentially unlimited number of Registration Bank files, although of course you can save only a certain number per USB flash drive and will need to swap to a different flash drive to save more.
That Play List feature also looks very interesting and powerful, since (as I understand it) a Play List record can call up any Registration from any Registration Bank file, as long as the appropriate USB flash drive is loaded.