The primary advantage of running the 64-bit version of the SOSS service on 64-bit Windows is that all physical memory on system is available to the ScaleOut StateServer service process. The virtual address space for 32-bit Windows is 4 GB in size, and it is divided into two partitions: one for use by the process and the other reserved for use by the system. When running the 32-bit version of the SOSS service, only 2 GB of virtual memory is available. It can be boosted to 3 GB by using the /3GB switch specified in the Boot.ini file, but to utilize all available physical memory on the server, running the 64-bit version of SOSS is required.
The following MSDN article,
Memory Limits for Windows Release has additional details on physical memory limits for different Windows platforms.
