is your system socket 754 or 939? with 754, that works just fine. with 939, you can technically use 3 sticks, but it will run in single channel and that will lower performance, since part of the performance rating of your chip assumes you use dual channel (generally around 300 performance rating points). some 939 chips (older than venice) even drop down to DDR333 and 2T command rate if you have 3 or 4 sticks installed. if you have a 939 board, i recommend getting a 2GB kit of similar memory to the sticks you currently have, and try to run all 4 sticks in dual channel, or sell the 512MB sticks and just run the 2GB kit (if you have something older than venice).