Windows 7, Vista, and XP 32bit will only use 4GB of RAM, its a fact. So, shoving 16GB in a computer running 32bit Windows will only use 4GB ACCORDING TO MICROSOFT even though 'System' control panel will show 16GB.
Please do note: Vista and Windows 7 on 32bit will report the TOTAL PHYSICAL RAM you have in System, but it will ONLY use 4GB of it. There might even be a 'hack' floating around to let you use more than 4GB, but I don't remember. The following table specifies the limits on physical memory for Windows 7. Some say to use PAE, which will allow you to address higher than 4GB BUT Microsoft will NOT let you on many of 32bit versions of Windows. The answer to this question is really 'depends on who you ask'.
