While you can do this if you wish, everything you just said is a bad idea.
For your memory to work in dual channel mode, you have to have 2 identical sticks in each bank. If you add a single 2 gig stick to the second bank, your memory will not run in dual channel mode. Mixing brands is a crap shoot. Sometimes it will work, sometimes it will not.
You want to add 2 more sticks of 1gig memory. Look for memory with timings as close as possible to what you have, and most important is the VOLTAGE. You must, must buy memory that runs at exactly the same voltage as what you have for the best chance of no problem.
Best thing to do all around, just buy a 2 stick 4 gig kit and replace the 2 sticks you have now.