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Any SSD will work but keep in mind that the G5 has only SATA I ports, which have a maximum real world bandwidth of approximately 135MB/s, so the fastest SSDs might not be worth it.

Of course you could get an Intel SSD which would increase the reading performance of small files but the G5 won't be able to reach the maximum sequential read speed (250MB/s) of such a drive.

Personally I wouldn't put too much money in such an old system, hence go with a cheaper SSD.
 
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