Ok, the thread is a little confusing for me (I'm slow today)
You currently have 1x2GB. A good offer to 2x1GB, if you go in that configuration you will get 3GB single channel since the IMC only can run in one configuration single or dual channel.
Now you say: Is mixing 2x2GB + 2x1GB is a good idea?, that means for me that apart of the 2x1GB good offer that you find, you will buy another 1x2GB stick RAM right?. If i'm right you will have 2x2GB + 2x1GB, both running in single channelas malmental says.