I believe it was tomshardware.com that did a complete analysis of both XP and Vista a few days ago to compare the performance of both OSes. The result was that Vista tanked on gaming performane, mainly because the graphics drivers and even the games themselves have not been optimized for Vista. However, audio/video encoding, file compression, and other multimedia-related tasks completed faster under Vista, some considerably moreso than others. Thus, it will seem faster in some areas and slower in others, but overall it will average out to about what you have now. Just keep up with any updates for your various software and hardware as updates should be forthcoming, improving the Vista experience.

John