Joker:
If you have two 1 gig sticks now, with 2 empty slots, I would buy 2 more 1 gig sticks.
We don't know for sure, but you presumably have standard speed RAM (5300) and so should buy more of it:
2GB kit (1GBx2), 240-pin DIMM, DDR2 PC2-5300 memory module
CT2KIT12864AA667
* Module Size: 2GB kit (1GBx2)
* Package: 240-pin DIMM
* Feature: DDR2 PC2-5300
* Specs: DDR2 PC2-5300 • CL=5 • Unbuffered • NON-ECC • DDR2-667 • 1.8V • 128Meg x 64
You will see it in the listing under 2 gig kits. I think it is 59.99.
If you think you might eventually go to 6 GB, you could buy a 2 gig stick now and another 2 gig stick later. These are not matched (in size) with your current 1 gig sticks, but are the same speed and certified by Crucial to work OK. If you are going to not go above 4 GB with this motherboard, I would get 2 more 1 gig sticks.
Here is what it says at Asus for your motherboard:
Dual channel memory architecture
4 x DIMM, max. 8GB, DDR2-800/667/533, ECC and non-ECC, un-buffered memory