From my own experience with that era of Asus motherboard, it seems it has issues with populating all 4 slots with the fastest memory if it isn't on the QVL.
1) DDR2 is almost obsolete and demands a higher price than DDR3 now.
2) Always try to have all sticks of RAM identical. And install in matching pairs.
3) I would replace the 2 - 1GB sticks with 2 - 2GB sticks. Yes, 4GB is the max the 32 bit OS can see. (you can sell the 1GB sticks on ebay)
4) 4GB will make gaming a much happier experience.
5) If price is an issue, I would forgo the 'sexy' memory sticks for some workable basic RAM. You won't see any difference in speed with the 'ugly' RAM. http://www.newegg.com/Product/ProductList.aspx?Submit=E...
Unfortunately, DDR2 and DDR3 memory is not interchangable. The slots are keyed different so you don't 'accidently' try to put one kind in the other's slot. If you are going to keep that motherboard, you will have to find and use DDR2 memory. Yes, 240 pin. The PC 6400 is just the 800MHZ designation.