Depends on what you use your system for. I've got one with 2 gigs, and one system with 4. I don't notice that much difference in speed when online or doing normal tasks in 32 bit windows. Adding 2 more gb, any windows 32 bit will see only about 1 gb of it. If you still want to try, look for the same ram type; many oem sticks are single sided, meaning the chipsets are only on one side of the stick. 667 ram is still available, or check craigslist for 2x2gb kits. If your board isn't on asus website, it's probably oem made for dell or hp. Those boards are listed on dell or hp's websites and are pickier about upgrades.
Your overall system will run at slower memory speed. It will run at 667 Mhz although you put 800 Mhz. To get 800 Mhz, you need to set to 800 Mhz in BIOS, thus, it will overclock the 667 Mhz sticks to match the 800 Mhz speed.