Phenom X4 will run with DDR2-1066 on RD790, AM2 will work up to DDR2-800 only

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According to various sources, AMD Phenom X4 will run with DDR2-1066 mode on RD790 while AM2 will work up to DDR2-800 only. It is not difficult to guess that the AM2+ Phenom series have a better memory controller that supports the higher memory bandwidth type.

Likewise, if you were to run a AM2 processor on a AM2+ compatible board, you will not see the option of DDR2-1066.