The steps you should take are:
1) go to
www.belarc.com/free_download.html
and get a profile of your system
2) go to
www.crucial.com, let it scan your system and make recommendations.
I did this, took Crucial's advice, ordered more RAM from them (a matched pair, very important) installed it, worked first time, PC now quite a bit faster with multiple windows open.
One satisfied customer (me) !
P.S. While you have the box open, give it a clean out. My cooling fans looked like they had flown thru volcanic ash.