xp uses less ram, cpu ect... but the way vista written my cause some games to run better with different hard ware. and poorly with other hardware. it mostly depends on your system and the drivers available for your hardware on the OS you pick. and that doesn't mean just cuz you have the newest stuff ether.... better drivers will be out for vista in time but for now ill stick with xp pro.
giving that Vista Transformation a go. seems okay so far, still uses up more system memory and resources; then again, I was running a barebones OS before....