Question: can i use 1gb and 2gb ram together?
Answer: No, it won't cause the world to implode, or any problems for that matter. Make sure the 2x1GB and the 2x2GB are installed in corresponding banks to enable dual-channel memory mode. Yes you can, but personally, I wouldn't. Ram is so cheap now, you should just dump the 1GB sticks and get another 2x 2GB.