Go for XP 64 as i dual boot 32 and 64 on AMD 4200 2GB MEM and Intel Q6600 with 2gb mem and 64 handles memory for 32 bit apps better.
I can render larger scenes with 64 over 32 and id give vista a miss unless you get vista 64 for its Internet security, as lots people dumping vista for xp that's why xp prices on ebay have sky rocketed.
Like 98ME and windows 2000 vista is a stop gap system to be replaced by windows 7 in 2010 so buying vista will be a waste of money.
I no people with super systems for gaming and 3d art ect who have dumped vista and gone back xp.