You may want to make sure the reason of your system sluggishness is not caused by malware. I will run first CCleaners & Malwarebytes Anti-Malwares first (you can download those for free in Cnet.com).
Then if it looks good, you can go on www.crucial.com which will scan your system and tell you the proper upgrade for your system, so you don't buy or install the wrong one as it will destroy your system.
Make sure you follow proper installation which you can find on ProfessorMesser.com
Good luck!!