With x86, if you add more RAM, you'll be limited. With x64, if you add more RAM it will use it all. However, x64 is a bit more temperamental in terms of less support for hardware devices (in terms of drivers
) as well as new and funky way of doing things (for those who have never used a 64bit Windows OS before).
Personally, for future-proofing purposes, I have to say +5 to the above posters - x64 all the way, baby!