Wait til tomorrow
With the latest Page 1 rumor creating quite a stir, I'd say wait until tomorrow to see if the fabled price drops/speed boosts are real or not. If so, perhaps you can get either the 1.6 G5 for much cheaper, or a dual 1.8 GHz for not too much more, relatively, as that would be a SWEET machine.
In general though, the way things are right now, I'd have to say that I see the 1.6 G5 as somewhat crippled by Apple. It is utilizing the latest and greatest chip, yes, however it has slower RAM, slower PCI, and other little things which make it not as desireable as the rest of the G5 family. To me, the G5 PowerMacs should be cutting edge, and as futureproofed as possible, and the 1.6 doesn't seem to quite fit with that philosophy.
So, right now, I'd say get the G4 dual and go crazy on RAM. But wait until tomorrow and see what
www.apple.com brings.

And if you're not in a big rush, I would strongly bet that speed boosts, and possible new configurations (i.e. dual 1.8s) will definitely be announced at MWSF in mid-January - but it all depends what kind of a rush you're in. regardless though, you won't be disappointed with the DP G4s, someone correct me if I'm wrong....