Ok, it's not the greatest performance tweak in the world, but anything helps.  This basically turns your flash drive into a spare pagefile for Vista to use. 

All you have to do for this one is simply plug in a usb flash drive, and when the prompt comes up, select Speed up my system.

If it doesn't prompt you to configure Readyboost to specify how much to use, you can open Computer, right click on your flash drive and click properties. 

From here, select the Readyboost tab, and you can configure how much space to use on the drive, or disable Readyboost whenever you want.

Now remember, this doesn't come close to replacing RAM, but it does help a little if you have a spare flash drive to use.

Also remember, the faster the flash drive, the better. 

Check out my full guide, How to speed up Windows Vista for more speed tweaks.