If you want slower that is up to you. The page for your motherboard is down at Asus at the moment, but your motherboard actually supports up to DDR2 1066,
but it is definitely not worth the price difference for the little bit extra speed.
By the way a couple of things to consider if you do go to 8 GB,
1. If you plan on upgrading the rest of your computer any time in the near future and had intentions of moving the memory, you won't be able to as all AMD and Intel boards now take DDR3.
2. Don't be surprised if you do take it to 8 GB and this happens, The system memory that is reported in the System Information dialog box in Windows Vista is less than you expect if 4 GB of RAM is installed
Note When the physical RAM that is installed on a computer equals the address space that is supported by the chipset, the total system memory that is available to the operating system is always less than the physical RAM that is installed. For example, consider a computer that has an Intel 975X chipset that supports 8 GB of address space. If you install 8 GB of RAM, the system memory that is available to the operating system will be reduced by the PCI configuration requirements. In this scenario, PCI configuration requirements reduce the memory that is available to the operating system by an amount that is between approximately 200 MB and approximately 1 GB. The reduction depends on the configuration.
Basically means that you will have somewhere around 7 GB available; could be a bit more or a bit less; depending on what is connected to the motherboard (which is pretty much everything from the video card to the sound card, etc).