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First off, what is does it mean to Match RAM or install RAM in pairs?
Matching RAM means that when RAM is installed, it is installed in two modules of the same size. This means when you want to install 1GB of RAM, it must be in two (2) modules of 512MB apiece. Further, they must be the same speed. Two 512MB PC3200 modules are considered matched, while a 512MB PC3200 and a 512MB PC2700 are not considered matched.
The PowerMac G5 memory architecture was designed to use matched RAM. If you own a PowerMac G5 of any configuration, then you need to match RAM modules. (Also see Knowledge Base Article #86414, "Power Mac G5: Memory Specifications and Requirements")
If you own a iMac G5, you need to match RAM modules only if you want to take advantage of the 128-bit data path. You can install unmatched RAM, and it will operate on a 64-bit data path. It is not a necessity to install matched RAM on an iMac G5, although doing so will boost performance. (Also see Knowledge Base Article #300082, "iMac G5: Benefit of using matching memory modules (RAM)"
On most other Macs, it is not necessary to match RAM, nor will it improve overall performance.
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