Welcome to Tom's Forum!
Assuming you have ASUS M2N-E -> http://www.asus.com/Motherboards/AMD_AM2/M2NE/#specifications
Dual channel memory architecture
4 x 240-pin DIMM, support max. 8GB DDR2 800/667/533 ECC and 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.
It depends on if you have a 32-bit or 64-bit OS, and even more important is if you're adding or replacing your current RAM. If adding the the 'best' is to duplicate the EXACT RAM that's already installed, next best is to find RAM with the SAME Frequency, CAS Timings, and same EXACT Voltage.
Here's a good selection -> http://www.crucial.com/store/listparts.aspx?model=M2N-E&Cat=RAM