there is a lot of eye candy and extra features you can live without (eg sidebar) in vista that you can turn off. That in effect makes it run just as good as XP plus you still have most of the vista functions (search bar, UAC etc).
-Try turning off sidebar. You'll notice MAJOR improvement to your system first go..
-Go to control panel > system > advanced system settings and customize the visual settings. Half of that i've got disabled, you can probably find some features you don't really use or need and turn them off.
-Get a USB flash drive (or if you got an internal card reader, then a card) thats Readyboost compatible. 2-4Gb or more drive/card if you can afford it, and it helps it somewhat.. but this option isn't as good as buying extra ram but hey, you can't take Ram with you to places and to store stuff with..
If you really want to go back to XP you will experience a faster system just because XP is rated to go on pretty much anything, even as low as a PII, and its been out for a long while, so everyone knows the benchmark so to speak. One other possible reason of going to XP (dual booting or otherwise) is for hardware or software compatibility that you need from it.