Just a couple of points from me.
First of all, let's begin on the increased Memory addressing within the x86-64 specification.
Micro-Processors of this age (IA-32 specification) are built upon a 36bit Physical Address, therefore permitting the system to address up to 64GB of Physical Memory.
However, 32bit Virtual Addressing is used. This therefore denies the system access to Memory larger than 4096MB (per application
x86-64 improves upon this by slightly increasing the Virtual Addressing size (through a combination of system rewrites and emulation), therefore allowing an increased Memory Addressing capacity.
Please Note: x86-64 Micro-Processors are not 64bit processors, as they do not conform to the IA-64 specification (or equivalent). Instead they "translate" the calls into 32 bit calls to be processed, with a performance penalty of course. For those of you who do not fully understand this it could be compared (to a degree) with what the Creative Alchemy application does.
Currently x86-64 security is much greater within an x86-64 operating system, however if hackers decide to attack the x86-64 market they will find it much easier to gain control over the operating system.
With IA-32[E] the system ran on its primary design, with x86-64 one of the crucial parts of the system is the EM64T instruction set (or equivalent). However, it is much easier to hack into, and control, an instruction set that is not a primary part of the system design than hack into the Micro-processor system itself. Instructions like SSE are not as vulnerable as they use the primary system to operate (unless under x86-64 mode). Please be aware of this high rated threat in the future.
The Kernel security system in Vista is a nice additional, however it will not be impregnable for much longer, as soon as they use a rotating modulation for the encryption system then I will be satisfied
Currently x86-64 is not a bad choice, however, for the future I would consider waiting for IA-64 Micro-Processors, the choice is yours.