One of the main worries people have when considering about upgrading to Windows Vista is performance. Is it worth it to sacrifice a responsiveness for a fancier GUI and more under-the-hood upgrades? According to these benchmarks, if you've got a relatively decent Vista install you actually don't suffer a large performance decrease.
In SANDRA 2007, Cinebench, Sciencemark, and 3D Mark 06, performance differences were had a change of 5% or less—some of which were actually better for Vista. In more real world examples, using Nero, LAME, and Sony encoders, Vista performed only a few percentage points slower than XP.
The moral of the story? Vista's not all that much slower than what you're running now. Which is pretty much what we've noticed on our own XP to Vista upgraded PC.
Windows Vista System Performance Reports [Techgage]