Not knowing how much RAM you have installed, you did not state it, the specs of the 435MT are up to 24GB of DDR3 1066 non ECC RAM. And yes it is a good idea to buy a matched kit for the best compatibility. Crucial has that RAM at their website here.
dell 435mt ram I need to increase my ram from factory spec to maybe 2x What advice is available to source the correct compatible ddr3 sticks considering the age of this computer I believe they have to matched? I have cpuz installed if that helps    
Studio XPS 435 MT, ram