Win32 will happily 'see' over 3GB, less than 4GB since the memory space is mapped for devices and so, depending on graphics card memory size etc. you'll get less than 4GB. 3.2/3.5GB for example. It can be made to see more than 4GB but you need to hack and recompile the kernel to do so and then make all applications compatible with this so not really of much use.

The 3GB 'switch' will allow such 'aware' applications to use more than the 2GB memory space allocation but can have side effects.