Desktop- RAM Upgrade Query?

best advice is to stick both ram chips in and boot the computer. If one is not compatible with the other the computer will make a "beep" sound over and over again and not boot. if the computer boots up and counts the correct amount of ram your good to go. they say not to mix and match the different ram speeds but in my experience if it the same type "ddr1" and "ddr2" sticks in the same machine it seems to run just fine. so if the ram is the wrong kind the computer will "beep" and not boot up and keep on beeping to let you know it doesn't like the ram. you wont hurt your system in doing this. im 99% sure of it.