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You do not need to configure the equivalent of /3GB while using a 64bit version of Windows. A 64bit version of Windows will even assign a 4GB virtual address space instead of 3GB for 32bit applications compiled with /LARGEADDRESSAWARE flag according to Comparison of memory and CPU limits in the 32-bit and 64-bit versions of Windows in this article.
Have a look at this KB article; it was written for repairing non-working dual boot configurations, but it contains instructions to create additional boot entries for previous operating systems: http://support.microsoft.com/kb/919529/en-us
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