I am running XP x64, and it
is 64-bit as it puts the processor into Long Mode and runs from it that way.
32-bit programs run in WoW, which is just an extrapolation of
Compatibility Mode, in which is a
submode of
Long Mode that preserves a full 32-bit x86 processing environment without actually having to formally switch processor modes.
The only programs I could not get to run are 16-bit ones (e.g. COM files), and that is usually done better in DOSBox, Bochs or VirtualPC anyhow.
Drivers are a PITA, but things are getting better since 64-bit Vista has been forcing manufacturers to face reality. I wouldn't expect some obscure webcam or printer to run. Logitech has been particularly annoying, but my webcam does work now. If you have a SATA drive, you'll want to get the 64-bit version of your MOBO/SATA driver onto a floppy disk for the installation.
One thing you can expect to lose is the context/shell (i.e. desktop right-click) menu entries from 32-bit programs. You can regedit and add them manually or find their 64-bit equivalents.
Avast anti-virus has had 64-bit support for the longest time, and were the first quality developer to do so if I am not mistaken. The home edition is free, lightweight and does its job with scanning and integration (e.g. e-mail clients) quite well. Norton used to be quality, as well as McAfee and AVG... but any time you have a software company that starts renting software to you... it is time to throw their garbage away since they have fallen into the hype+bloat category due to profit margins becoming more important than quality.
7zip has a 64-bit port. I would drop WinRAR/ACE/ZIP and get used to 7zip even if only for the great compression ratio of LZMA and the various options. The GUI is not the best in the world, but it is straight-forward and the context/shell menu as good as any other.
For whatever reason, with 32-bit XP on this computer, it would take Counter-Strike Source nearly 3 minutes to load. With 64-bit XP, it took less than 10 seconds. Even though some programs might be 32-bit, the operating system and core libraries are 64-bit and that allows for faster file I/O along with more CPU time for all other processes. The worry about 64-bit programs having larger addresses resulting in more bytes per instructions seems to be quite unfounded in terms of reality.
All-in-all, my experience with XP x64 is that it feels more light-weight than your default install of 32-bit XP. I lost the capability to run 16-bit programs, but I can freely develop and run 64-bit programs... which seems to be much more interesting anyhow
I have love in command line and old skool programs and I dislike the fancey 3D shiny desktops etc., no need to say that 65% programs aren't running properly in vista, plus games are having bigger system requirements (remember the Windows's installations?: "This version of Windows will make your PC run more faster"... yeah right. 
) -> So I'm planning to de-Vista the PC 
, repartition the harddisk of my own and to install the good ol' XP. As I've mentioned I like older applications and the CPU in the new PC is dualcore amd athlon 64bit... Would it have negative impact on speed and performance if I would install the 32bit version? Or it would run much better if I would install the 64bit one? for example STALKER or Crysis... But what about the legacy applications? I just can't say goodbye to them (I know XP x64 has MS-BOSS emulation like XP 32, but some applications aren't running in x64). Of course if the games would run better in 64bit version 
, I would install that.
