You can configure a Mac Pro with two Core 2 Quads and up to 32 GB of memory.
Yup... Apple beat Microsoft to desktop 64 bit processing with 64 bit G5 PowerPC processor and OS X Tiger. Since April 2005, Tiger is the only OS needed for the Mac platform regardless of whether it has a 32bit or 64bit platform. With 32bit CPUs it runs as a 32bit OS. With 64bit CPUs it runs as a 64bit OS. Even before Tiger, OS X (on 64 bit PPC processors) recognized more than 4 GB of system RAM. Individual applications were, however, still restricted to the 32 bit memery space.
64 bit processors are becoming the norm. However, unlike rgeiken implies, 64 bit Vista is still in the extreme minority among operating systems. And while Core 2 Duo and Core 2 Quad CPUs are being pushed by Intel, many, many thousands of Core Duo (32bit), dual-core Pentiums (32bit) and newer dual-core Celerons (also 32 bit) processors being sold. For those 32bit CPUs people still require 32bit Vista. As a matter of economics (many millions more copies of 32bit Windows than 64bit Windows), 32bit drivers are still the norm. 64bit drivers, while becoming more available in the Windows world, are not so much the norm.
The same holds true for applications on the Windows side that take advantage of 64bit memory space or the wider data path. As well, most mainstream motherboards (Dell, HP, Acer, Gateway, and eMachine) won't handle more than 4 GB of RAM reguardless of what the OS or CPU can use. Since HP, Dell and Acer (Acer owns Gateway and eMachine) represent the top three PC makers in the US, what they offer has a great bearing on what's available in the market.
And least we forget: The fastest Vista laptop in 2007 was the MacBook Pro. But then that's about hardware platforms and (oh, my!) Microsoft doesn't make computers (and no, the X-Box doesn't count). Also, it remains that while Windows owns 90% of the OS market. While that's the lion's share, it's down from the same time last year. No matter how you slice it, that's still a smaller (or less huge) piece of the pie for the lion.
I haven't consumed too much Apple Kool-Aid. I'm a Windows user on four of the five computers in our household (one with Vista, the others with XP). I'm also an IT tech for a school district with something on the order of 1500 PCs and one (1) Mac. But I'm realistic enough to understand Vista is a dog-and-pony show trying to pass itself off as an OS update.