We have been doing some initial benchmark testing on Windows Vista using PerformanceTest V6.
While the results are very preliminary we have found that Vista seems to be about 20% slower than XP. This seems to be mainly due to the 2D graphic tests and more surprisingly the memory tests.
It is no surprise the 2D graphic tests are slower with all the extra eye candy in Vista. But at the moment we don't have a good explanation as to why the memory speed might be effected.
On the plus side the 3D results were slightly better. Maybe due to the new version of DirectX in Vista.
The CD test failed to run on Vista. We need to investigate this.
So anyone getting Vista will need better hardware to get the same performance (in particular a better graphics card).
Link to all the detailed results