to my understanding (http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778(v=vs.85).aspx#memory_limits) WIN 7 64bit edition can address RAM in the range of:
Home Basic: 8GB
Home Premium: 16GB
There is Ultimate on this rig, which is fitted with 32 GB of RAM and 8 GB of VRAM, which neither can be addressed by SU with a rendering plug-in like Thea. And thus SU 32bit being a bottleneck, also to my understanding addressing only a single CPU core, even on multi-core rigs.
There is plenty of RAM on the rig and no issue with paging file or any encryption. There are certainly some other 32bit apps running (eating out of the 4 GB limit) and thus SU does not make it to the full 4GB but starts freezing at around 3 GB, not recovering and can only be closed by crashing it.
The SU model is about 75 MB and when opened SU is committing around 650 MB of RAM. Starting the rendering process with the Thea Render plug-in SU is bleeding RAM up to 3 GB and then freezes - as SU being a 32bit app and having reaching the address space limit.
From other threads in the net it seems that the SU developers are aware of the matter but not taking to develop a 64bit SU version… Which is strange as the app (Pro) being marketed as a professional 3d modelling tool, yet neglecting the need of a 64bit version.