Microsoft funded a study by Principled Technologies to compare the performance of the Windows XP user interface to the Windows Vista Aero interface. Here’s my summary:

• Comparisons were done with a variety of different computers, some more powerful than others. Naturally, the Aero tests required computers capable of running Aero. So, these results don’t necessarily apply to your old XP computer.
• With or without Aero, Vista performs about the same on business tasks (like opening Office documents). Some things are faster, some are slower, but it’s a wash overall.
• Vista with Aero performs about the same as Vista without Aero. So, don’t feel like you need to disable Aero for performance.
• Though all my computers are capable, I can’t run Aero because screenshots look awful. I’m fine with either interface. I’m just saying, even if Aero did cause a performance impact and you wanted to turn it off, the other Vista improvements more than make up for it.