How many slots do you have? I presume there were two sticks of ram? What operating system are you using?
are you running win 7 64 bit?

Last time I added more ram I used this site, if you download the scanner it will tell you all you need to know, very small download and perfectly safe.
http://www.crucial.com/store/drammemory.aspx
And maybe a new processorI guess it's time I got a new mobo.