That would be hard to tell for sure. RAM sometimes has compatibility issues with other RAM if they are not the same make, model, size, and so on. If you did try to put the 1GB ram in with the 2GB stick you would have to change the clock speed on one or both sticks of RAM. Computers run all RAM plugged into them at the same speed, so you will have to either underclock or overclock one of the RAM sticks or change the clock on both so they are synced up. It would probably be best to adjust one as close to the other as possible before inserting the second one.
Even with changing the clock speed, it is still questionable at best they will work together. Chances are they will not. You would have a lot more success if you attempted two RAM sticks of the same clock speed.