Hello synth could you run Speccy and post back the motherboard section opened out so we can see the chipset version please? Speccy - Download
< from Piriform
As a general rule you can put in a faster RAM stick but the speed depends on the chipset and what speed RAM it can handle. So for example if you put in 1600MHz RAM sticks and the highest speed RAM the chipset can only handle is 1333MHz RAM then the 1600MHz will be throttled back to 1333MHz - in a nutshell. My Sandy Bridge desktop is an apt example it has 16GB of 1600MHz in but can only use up to 1333MHz of the sticks speed.
You are far better off getting the same speed RAM and buy two sticks as pair then they are more likely to be evenly "matched / balanced" when in use.
My Toshiba is running 8GB of that speed yours is and it is just fine. If it is performance you want then try this Optimize Windows 7
you lose some of the "bling" but it will run far more efficiently.
As it says in the tutorial a SSD will blaze your mind away LOL!! - if you don't store a lot on it and if you do you can always dump the extra onto an external drive which by the way you ought to use as a back up anyway for emergencies