Not only is Vista not exactly setting even high-end gamers' hearts afire, but Blizzard is traditionally about as lenient as it gets on system requirements. They did up the ante a little bit when Burning Crusade was released, and we do know that Wrath will require a little more of your computer with those shiny ice shaders (not too much more -- they may ask for more RAM or a faster than the current 800mhz processor). But as much as Microsoft would love what may again be the best selling expansion in the world to require Vista, it's just not going to happen -- Blizzard has too many subscribers here to lock a good number of them with an operating system that's not going over so well.
So ignore what your smirking friend tells you -- if you can run Burning Crusade, you'll be able to run Wrath on the same box.
Microsoft Windows Vista