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If you could tell us what kind of PowerMac you have that would be more helpful.

If you have a PowerMac G5, you'll have to buy 2 of those 512MB DDR RAM Sticks as you have to install the RAM in matched pairs because the PowerMac G5 uses dual channel memory.

So yes, that RAM will work fine in a PowerMac as long as you buy 2 of them. That will give you a 1GB upgrade, or 2x512. If you just want a 512MB upgrade, then you'll have to buy (2) 256MB DDR400 Sticks.
 
Oops, sorry....they will be going into a dual 2.3, so yes, I was looking into 2 of them. THanks!
 
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