Having used every windows from version 3.0, XP 64 sits on my dual core and quad core machine with XP32.
No i ignored the stop gap windows 98 me and 2000 though did manage get the 98 defrag of 98ME as its bloody faster than 98se one, so i wont touch vista with a 10 foot barge pole as i will buy windows 7 in 2 years which will make vista obsolete.
No wounder gates keeping XP 64 far longer now as he don't want people go mac, and i have used vista on various machines, and find xp better.
Though if you have to use vista especially for the Internet you must buy vista 64 as it has more security features than vista 32, so if you use 6 vista to surf online ya far safer than using 32 vista online.
Got a friend to try with both versions while he on holiday, on 2 spare machines to download from net and to have spyware install to both machines.
He said as i said he found vista 64 would work with some spyware that would make vista 32 slow down or not boot, so viista 64 is the clear winer against vista 32.
Just like xp pro is more secure than xp home and i have 2xp pro and 3 xp home.