Anything today is 64-bit. ML is also 64-bit kernel. It was only pre-2010 models where there was an issue and between 2008 thru 2009 as Mac transitioned to a full 64-bit firmware.
No on the upgrade. Yes on "System Builder" edition (like Home Premium 64-bit SP1 $99).
In October Windows 8 hits stores $39 and you can get a taste using the Preview or the 90-day Enterprise that is the Releast version but expires - so you could go free and then $39 later.
Microsoft Windows 8 Previewhttp://msdn.microsoft.com/en-us/windows/apps/default
Windows 8 Blog
Installing Windows 8 On a MacBook Air
Without SuperDrive on older Macs
The download size of Windows 8 RTM x64 (English) is 3416 MB and x86 is 2511 MB.
-------Posted September 15, 2011 – 5:37 pm in: Windows 8 Guides
For all those Mac users who have been eagerly waiting to test drive the next generation Windows 8 operating system on their Mac, here is the simple procedure that you need to follow to install Windows 8 on your Mac using the Boot Camp Assistant software.
We have guides to help you install and setup Windows 8. Check out following guides while waiting for your ISO files to download:
Windows 8: enable Hyper-V, it really helps.
Intel Windows 8 Graphic Driver
Intel Download Center: http://downloadcenter.intel.com/
3rd Generation Intel HD Graphics 4000 Windows 8 64-bit
MacBook Pro Cirrus Logic Audio device driver
Changing this setting fixed my brightness problems on my MacBook Air running Windows 8 through Boot Camp.