I wondered if your initial query was going to be followed by, "Why".
1. IF, there is 4G RAM or more in the machine, x64 will see & use all of it. x86 will cap out @ about 3.25, no matter how much RAM is actually (available) in the machine. Still, even w/out 4+ RAM, x64 is THE way to go... & if, the RAM was ever increased, the OS could use it (all).
2. There is some school of thought that x64 is faster & more stable than x86. Generally, x64 is better @ strenuous multi-tasking.
3. x64 is definately the future of computing. In the not too distant future, x86 will be a thing of the past.
These days I always recommend to clients, that if, a machine will support x64 & if, one wants to be able to access all available RAM then, now & looking ahead, absolutely go X64.