You want equal amounts of ram in each channel. So, check in your MB manual to see which slots are channel 1 and which are channel 2, and then put 2GB+1GB in each channel. Are they the same rated speed/timing/voltage?
If you have a socket 775 board, and are trying to use more than two DIMMs, you will need to 'bump' the MCH voltage by a tenth, or a tenth-and-a-half, to accommodate them. Realize that, if you are doing this, and running memory faster than 800, which is already 'overvolted' (likely, 2.1V instead of 1.8V JEDEC), you are both over-volting and over-clocking your MCH - stability may require MCH cooling: