Wont matter either way, as it'll still be running in Dual Channel mode.
For instance, I have a Dell that has 1.5 GB of RAM, it came with 512MB in 2 256 MB modules, and I dropped in 2 512MB modules.
It says its running in Dual Channel mode. So, it wouldn't matter if I pulled out the 512MB that originally shipped, as long as I don't go over the 1GB mark in usage, it'd be the same, no matter which way I did it.