Windows Vista Performance Information Utility
Once you get Vista installed or you purchase a new Vista machine, open the new Performance Information and Tools in the Control Panel. You should see your "Windows® Experience Index" ... I would suggest taking a screenshot and saving it. Then as you add/change drivers or hardware you can re-calculate the WEI, however once you do this the previous score is gone.
The reason I mention saving the screenshot is once I updated my graphics (ATI) card drivers, the "Graphics Subscore" dropped from 2.9 and 5.1 to 1.0 ... ouch! Needless to say I removed them and went back to the (Vista) supplied drivers ...
You can also use this utility to see if Overclocking your system is really doing any good. Again take a screenshot before and after. I jacked up my AMD setting in the BIOS and a few other setting without getting too carried away and was able to gain quite a bit, and Vista is still very stable.
You can find a very good article here on the "Windows® Experience Index".