I use Unity to manage and display very heavy data.
The build run in windows 7 x64 and can use until 3.2GB of RAM.
So, since the version 3.2, standalone build can't exceed 1.5Gb of RAM ( like windows x86 system ).
You know, this is a bug or a new feature of new version of Unity ?
Futur version will have the same 'problem' or not ?
Since Unity is still a 32 bit application, memory address space is limited. However the 1.5 GB RAM threshold I haven't heard about. Maybe you're cramming too much stuff into your video card memory? How much RAM does your video card have? (But then, wouldn't it swap between main memory and video memory?)
Once Unity goes 64 bits, you'll be able to utilize more memory. When they do, I do not know.