First, your motherboard must support 8gb of ram or more.
Secondly, you must install on your machine a 64 bit operating system to recognize all 8 gb of ram.
Any 32 bit windows will only recognize 3,25 gb of ram. Do the math required and you will see.
Windows Vista and 7 on 64 bit can recognize 128 gb of ram. Search the internet a bit for more info and you will see that i am right.