Install just the new stick. If it crashes then the new stick is not compatible with your system.
I have a Dell Optiplex GX150 1Ghz with 128MB double-sided SDRAM (133MHZ) installed. I was wanting to install more RAM so I saw a single-sided 128MB SDRAM 133MHZ stick on ebay. I bought it and installed it.
The computer recognized it and upped my memory total to 256MB, but crashed during startup, sometimes displaying a number in Hex that I assume was a memory address, other times just stuck with a blinking cursor in the top left hand corner of the screen. It crashed whether it was the only memory module in the computer (in the first and then the second slot), and whether it was in the first position and the other module was in the second position or vice versa.
My question is, did I get non-functioning RAM or would my system not support single-sided SDRAM?