I've tried windows Vista rc1 on my PC it's okay, but to me it seems like Microsoft copied a bunch of Mac OS 10.4s features, edited them to to look more windows-ish, and somehow made them require much more ram than those same features in 10.4 does on my powerbook g3(fast with 384mb).
Even if you switch vista to the classic look it still takes way too much memory.
The supposed added security, it'd be cheaper and IMHO better to buy win2k(my favorite windows) and download AVG, ZoneAlarm, and AdwareSE/SpyBot.
