As cars get on in years they can start to nickle and dime you, not that a 2003is that old mind you.
It you feel it's time to move on by all means go ahead, I doubt there any big changes in store for 2008. I think you'll notice a more hushed and comfortable ride, plus sharper styling, (at least from the rear view
).
However, unless there are certain options you just can't live without, the DTS I is the better buy. Levels II and III add unnecessary expense and complexity in my opinion, (more stuff to break down).