My experience:
2.8ghz w/1gb ram. Took a spare hard drive and loaded Vista on it. I could not play a DVD on this computer. Vista was hogging too much resources. Eye candy was turned off.
Slapped Vista onto another computer (2.5ghz/1gb ram) and it crashed with rebooting 5 times in 4 hours. Same computer works fine with XP
Then thats pretty abnormal, actually very abnormal. I have a friend who is running it on a athlon xp 1800+ with only 512 mb of ram w/o a hiccup and the Vista at most is only taking up %40 of system resources, when he plays a game those resources are released to the game. ANother guy got it to work decently on a 1 ghz computer with 384 megs of ram.