Dual channel memory architecture
4 x 240-pin DIMM, support max. 8GB DDR2 800/667/533 ECC, Non-ECC, un-buffered memory
*When installing total memory of 4GB capacity or more, Windows 32-bit operation system may only recognize less than 3GB. Hence, a total installed memory of less than 3GB is recommended. ASUSTeK Computer Inc.
That comes right from the specifications seen at Asus for that model board showing a maximum of 8gb of memory in 4 dimm slots. When you need information the best place is always the manufacturer's own site. Often vendor pages will have misprints on things.
The ideal but not always the mandatory is seeing all dimms comes from the same manufacturer's batch of memory and pretested together. In real world terms often brands are mixed as long as the type, CAS Latency, and voltages are the same.
The best with four dimms would be simply buying two identical pairs of 2gb dimms to see 8gb. The actual need for more then 2-3gb on a desktop is mainly for someone like a rocket scientist or software engineer using CAD or other "memory hungry" programs.
Even with the 64bit editions of Windows the softwares and games out don't come close to seeing a need for large amount of ram. In some cases that can actually become more of a performance hinderance.