I'm surprised no one mentionned this yet, but the CPU (Processor) has to support the x64 instruction set.
i.e. : The CPU has to be a Core2Duo or newer generation.
The CoreDuo does not support a 64 bit OS.
So, while drivers and EFI play a big part, don't forget to make sure your CPU is up to the task.
On a side note, I find it appaling that Microsoft has not gotten its act together and pushed for better/ubiquitous 64bit support yet.
4GB+ of RAM is pretty much becoming commonplace these days and anyone using a 32bit version of Windows cannot make use of the whole 4GB+ (the max useable amount is 3.12GB or something close to that figure with a maximum of 2GB allotable per process).
On the other hand, Mac OS X has supported more than 4GB of RAM for quite some time now and with Leopard, the whole O.S. is 64bit capable.
If you want to use more than 4GB of RAM on the Windows O.S., you need to get a separate version, and while it's less of an issue now, the 64bit versions are often plagued by a lack of drivers, late drivers or buggy drivers.
I really like the fact that with any Mac you buy these days, you get full useage of your RAM and 64bit across the board (future-proof) and you don't have to worry about choosing the correct O.S. version from 7-8 different options. You get the whole deal the first time.
Also, I live in Canada, more precisely in the bilingual (french/english) province of Quebec and finding cheap OEM versions of Windows in french from reputable dealers like NCIX is hard. They often only carry the english versions.
The Mac O.S., on the other hand, includes 20+ languages right from the start, and I can even set my girlfriend's account to french and mine to english (even on the fly) whereas with Windows, you need to buy a completely new licence to be able to change languages and even then, It's either french or english, you can't have both in the same install.
The only way to go to have both is with Vista Ultimate's MUI packs but Ultimate is considerably more expensive than, say, Home or Business.
/rant.
Sorry, back on track now.