XP versions?

Both are seperate OSes, you can get http://www.microsoft.com/windowsxp/64bit/default.mspx

but Normal XP is as Adryn said just x86, the above link is to a x64 version of XP.

 

x64 should in theory be much quicker in daily usage that x86, however a pitfall at present is driver and software compatability, the more that use x64 the more 3rd party companies are going to have to re-write their software for it, BUT users running x64 OSes are in a minority.

The more software is written specifically for x64 the more the speed advantages of 64bit will become apparent IMO.

Other noteable advantage of x64 is RAM memory support in large sizes over the 4GB limit in x86 Vista http://www.microsoft.com/windows/products/windowsvista/editions/64bit.mspx ( altho you would have to check your motherboard maker as some Bios's need updating )

For a user, is x64 worth going to yet, well for the majority NO, not enough compatible drivers or apps available yet to keep many happy, for the techy types then yes its worth a go if you have a second spare 64bit compatible hardwared PC.

I have used Vista Ultimate x64 and found it fine for my daily usage, but it will not be for all.

 

Mmmmm, No as my laptop which I first used Vista on was compatible with Aero just in having the Ati x200 GFX onboard and I also had 1GB Ram and an AMD64 3500+ CPU and that ran Vista Ult really well, but I always had in mind a new PC build for my desktop, which has these parts in it and they are all compatible http://forums.majorgeeks.com/showpost.php?p=911253&postcount=23


But I would advise not only running the Vista upgrade advisor but also not taking its advice too litterary, do your own research if your planning on installing Vista on a PC which is over 1yr old, especially for all hardware drivers.