You guys are also missing a very crucial point....
In 2005 when the C6 will be released, the new BMW 3 Series will be out and that introduces a new generation of BMW straight six engines. Which the Z4, 5 Series and 6 Series will get.
These engines will be more powerful, more economical and more eco-friendly.
So its very hard to judge....if you are buying the car in 2005 the Z4 will look the same but have new stuff under the bonnet, the engines in the Z4 were introduced in 2001, BMW update their straight sixes every 4 years or so.
The Z4 is still using the same engine the Z3 3.0 used and the new 5 Series is using that same engine.....as well as the old 5 Series...
The new engines will be a VERY welcome addition to the BMW arsenal.
Also romours are going around that the next generation 3 series will be available with the 3.5 V8 you can find in the current generation 735i.
So maybe the Z4 might get it.......not likely but its posible