Superfetch should reduce hard drive access, meaning lower average power consumption.
That is working on the assumption that the PC is on for sometime. If the PC is on for 5-minutes to check what the road congestion is like before you go to visit someone for the weekend, then XP will win hands down because its not thrashing the HDD for 5-minutes solid, it will load what it has to + your Web Browser.
The flip side to that is if you have your PC on all day in regular use, at which point Vista should end up winning/drawing.
Without an actual study we wont know for sure when Vista will break even with XP, but I guarantee you that on most PC's in existence made within the last 4-years XP will win. In a year or so the tables will be turned when the "average" hardware can cope with Vista + the mountains of crap software that people (and OEM's) install. This has happened with the transition between 95/98, 98/Me, Me/W2Kand W2K/XP.
I hope that MS have learned their lesson with the many Flaws of Vista and actually make "Windows 7" faster than the previous OS.
But to put all of this into perspective, the differences will be very small unless the PC is severely under powered.