So the 4 states of Ajax adoption are as follows ...
3. Second Site
The problem with progressive enhancement is that it treats the minority noscript case first. The second site approach provides the best it can for the majority and for the minority too. Google, Yahoo and Amazon create far more advanced systems with this approach. It's likely to cost a bit more, but you're probably going to get a much better site out of it.
You can work up the tree, starting out small and getting bigger, and the good news from a DWR point of view is that it can help you each step of the way. DWR is good for small tweaks and a more full on style.
What's your prefered approach?