Memory compatibility is not much of a concern these days.
With 4-slots on your motherboards, you have two independent pairs of dual-channel memory. Your 2x1GB dimms already run as single dual-channel pair. The other memory you install would run as a separate pair. The two pairs do not need to be matched (dimms in a pair should match for best results, but even thats not necessary, the size is the only necessary factor for dual-channel).
You could actually run your crucial ballistic 2GB kit with a PNY 1GB kit (2x512MB) or a Corsair 4GB kit (2x2GB), and even at different speeds if you wanted (though why bother).
If you want to double your memory, just find some that runs at the same settings: DDR2 800, 5-5-5-12 (or whatever) timings, 1.8V (or whatever). This is your best bet for maximum performance.