What is a Window Vista memory limit?

The 32-bit Windows Vista memory limit is 4GB due to 32-bit address space(2^32 = 4G). However, Windows Vista uses about 1GB of memory address space for another use, so there is about 3GB usable memory space. I.e. there is no reason to install more than 3GB of memory to your computer if you are using the normal (32 bit) Windows Vista. If you need more memory space you must buy 64-bit version of Windows Vista.

It is the x86 HARDWARE that uses the top 1 Gb memory. It is not a Windows problem

It is your graphics card memory and memory on other cards in your system that is subtracted away from the 4GB maximum. So if your graphics card has 512 MB, then the most RAM you can use is 3.5GB.