Linux can use 4+GB even on 32bit.
Windows XP cannot and is limited to 3 - 3.5GB max, even if you physically have 4GB or more.
Either way amd64 / x86_64 is definitely the way to go if your CPU is 64bit.
Practically all AMD CPUs are 64bit.
All Intel CPUs with EM64T / x86_64 extensions can run in 64bit mode.