Here is some useful information.
[amd64 or x86-64 is an extension of the IA-32 32-bit version of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on IA-32, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other enhancements. The original specification was created by AMD, and has been implemented by AMD, Intel, VIA, and others. It is fully backwards compatible with 16-bit and 32-bit x86 code.(p13-14) Because the full x86 16-bit and 32-bit instruction sets remains implemented in hardware without any intervening emulation, existing x86 executables run with no compatibility or performance penalties, although existing applications that are recoded to take advantage of new features of the processor design may see performance increases.
AMD's method of extending Intel's 32-bit x86 instruction set to be a subset of its x86-64 instruction set is the same technique Intel employed to extend its 16-bit x86 instruction set to 32 bits."]
I have decided to triple boot new 64-bit Ubuntu 12.04 LTS with existing (Ubuntu 12.04 LTS, 32-bit and Windows-7, 32-bit; dual boot) system.
Why not you?