@Bliq, doesn't the XPS 16 currently carry the 4670?
@OP: Is there a particular reason you want the XPS 16 over say, the Studio 15? Most of the hardware is the same, except you get the better GPU and a lower base price tag. You do lose out on the extra 0.4" diagonal screen size and their special LED screen though.
We won't know when Dell's upgrading the XPS 16 unless we work there, in which case we'd probably not be allowed to reveal that information. Try calling Dell's customer service and asking them?