Vista/CPU x64 vs x86 & RAM >= 4GB & Intel 975x?

Vista/CPU x64 vs x86 & RAM >= 4GB & Intel 975x?

My scenario...

mobo: ASUS P5WDG2-WS w/Intel 975x chipset
cpu: Intel Pentium D 960 3.6GHz 2/2
ram: 4 x 1GB (total 4GB)
video: Asus EN7950GX2 (512/512)

Which Vista, when combined with this hardware setup, will be able to map, access, fully recognize and utilize RAM >= 4GB ?

Which Vista, on this hw, will best utilize its 64bit capabilities AND RAM >= 4GB?

I've gone cross-eyed Googling documents from Microsoft, MSDN, HW and SW vendors and review sites without a DEFINITIVE answer.

What is the FINAL word on this?

Okay, let me take another shot at this.

Of the 1001 versions of Vista, which are available as x64?

Of the 1001 version of Vista NOT x64, have any of them broken the 4GB barrier?

Windows Vista: The Editions

... tells me the versions, but I still cannot dig up the information I need to know... or if there truly are x86 and x64 versions, or just x64?

Product Guide

... almost makes it look like x86 and x64 are one and the same; that Vista will automagically decide if you have a 32bit or 64 bit system ... can't be that good. 8)

... FINALLY! Some hard words from at one source (now to confirm), something I wanted to know, that I haven't found anywhere else:

Finally, all except Starter will be available in both 32- and 64-bit versions, with both packaged in the same SKU. "The bits will be in the same SKU," confirmed a Microsoft spokesperson Monday.

Woohoo! Confirmation it seems:

Windows Vista Feature Focus: 64-Bit (x64) Support

... so much trouble to find the right google keywords.