No it won't work (technically yes depending on platform). You need to exact same RAM chips to use dual channel mode. You should look for a dual channel kit.
Another thing too - you will need a 64BIT OS to see more than 4GB of RAM.
Read this great info guide for an explanation of Dual channel RAM and problems/issues with different ram sizes/speeds:http://forum.notebookreview.com/hardware-components-aftermarket-upgrades/167472-dual-channel-ram-guide.html