If you need a computer now and can find a discounted 1.6 on sale, I would consider going ahead and buying it now, although if you can wait to June / July I think it will definately be worth it. In any case, I wouldn't pay full price it now, as the prices haven't been updated in half a year.
Do keep in mind that it's possible that the price may very well go up for the base model when the new ones are released (i.e. dual 1.8 GHz for $1999), but as another user commented, you will probably be able to find the 1.6 at significantly discounted prices after the new models are released.
I was personally in this dilemma about 3 weeks ago - I had been waiting for the Rev. Bs since January - and finally gave up and got a 1.6 on sale for $1499. The machine is very fast and meets my needs as a Web / graphic designer; I can only imagine how fast the upper end machines are. Unless you are doing a lot of video editing / rendering, the current base model G5 should be enough for you.
And by the way, it's also quieter and consumes less power than the dual processor machines.
In any case, if you do get the 1.6, be sure to get a lot of RAM for it; the 256 MB that it ships with is a joke.
Hope this helps. 🙂