Before buying Windows 7 64, you should be aware...

[badge]

Maximum memory limits for Windows 7 64 bit versions.

The one to look out for is the 16GB limit on Home Premium. If you’re building a Core i7 system then it’s quite easy (if you have the cash!) to exceed this limit.

Starter: 8GB
Home Basic: 8GB
Home Premium: 16GB
Professional: 192GB
Enterprise: 192GB
Ultimate: 192GB

 

[LePhuronn]

Until 4GB DIMMs are affordable to the public you won't see non-enthusiasts running more than 12GB anyway so it's not an issue.