Don't change your RAM. It will be a waste of money. Plus, you go from 1600MHz to 1333MHz, which is a drop in theoric performance. Also, you won't see any real difference if you go from 4Gb to 8Gb. Plus, if you own a 32Bit system, the max amount of RAM your system will see is 4Gb. If you own a 64Bit system, and you don't use applications that need lots of RAM, I say stick with 4Gb.
If you really want (or need) 8Gb, then I'd go with higher quality RAM, like this one : http://www.newegg.com/Product/Product.aspx?Item=N82E168... It has an included fan to keep it cool if you overclock it, and has very low latency.
If that's too expensive for you, take a look at this kit : http://www.newegg.com/Product/Product.aspx?Item=N82E168... There is no included fans, and the heat spreaders are a little bit less efficient. But the timings are the same.
My recomendation would be to save your money for a better video card. Then you will see a real world improvement.