I run XP x64 as my main OS on my Ferrari 4005. I've been pretty happy with it, mostly because we are lucky enough that Acer released all the needed 64 bit drivers. Stability wise, I think the machine crashed a couple times in 6 months and it must have been the hardware, since I haven't seen a Windows' blue screen of death yet.
As a replacement for the regular XP, most of the 32 bit apps work (Firefox, Thunderbird, Nero, OpenOffice, iTunes -not the CD burning feature though- and games: Quake4, Call Of Duty 2, Civ IV), the system itself feels a bit more responsible and 64 bit apps are a bit more efficient. There aren't many 64 bit app out there yet though
The only one I use all the time is 7-Zip
, a free Zip/7z/Gzip/BZip2/... compressor tool. There is a sizeable speed difference between the 32 bit and 64 bit versions in my machine, verificable by the benchmark it features. neat
Be warned that a 64 bit system will use more memory, because the code instructions and data in programs are bigger. I read somewhere a 64 bit process use roughly 15% more memory. Not a big deal on these 1GB RAM days though.
By the way, some shitty installers don't really like this Windows version (weirdly, XP x64 is a derivative of Windows 2003 x64). By example, it took Google Earth a couple of updates to catch up and install fine, while it is still a 32 bit app