Whatever you decide will be governed by what the mainboard you have's compatability of it. Some boards will accept far higher speeds than others so check the models' version of your mainboard and ensure you have the latest BIOS version installed for it.
Worst case scenario is that if you do buy from Crucial and it is the wrong one after following their advice they will usually exchange it for you.
In my experience, and I have been building PC's since DOS was the only operating system (very old git and geek) the faster speed will not necessarily increase your PC's performance because it is not only the memory which affects the PC's speed. CPU power and HDD access speeds also affect it as does the general basic setup of how the BIOS operates and controls it. Then there is the 32 or 64 Bit versions with 64Bit being the faster.
First thing you should do is to completely clean out all of the bits and pieces of programs etc that you no longer use and do a deep Disk Clean, Registry Clean and a Defragment of the HDD.
Hope that it does not put you off but better to try to improve the performance before spending money out that maybe you do not have to.
You can also PM me if you need more advice
