|Is your Vista version 32 bit or 64 bit ?|
If it's 32 bit, Windows can' t recognize / use more than 4 gb of ram.
If you don't know, RIGHT click on the Computer icon and choose Properties.
If Vista is 64 bit it will show you that on that resulting window.
The 4gb virtual memory address limit for 32 bit operating systems.
An example of 3gb working better than 4gb in a 32 bit operating system.
See Response 6: