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My order should be here soon I hope I dont have the issue you had. Was the G5 like mine, same specs? Were you upgrading from 4GB to 8? Did you purchase all new 8GB or added to?
I had purchased all 8 GB, and it was a model that came with 1.5 GB onboard, although that was all that the system could detect. In reality, it had four 512 MB sticks inside.

Curiously enough, I did try to mix and match two of these sticks into the system when it became clear that I wouldn't be able to max out all 8 GB of my RAM. I thought it would be fine to settle with just 7 GB, but that didn't pan out, so now I'm only using 6 GB.
 
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