The problem is that some Macs are very picky about RAM. In my experience, there is only one place that I trust to buy Mac RAM from, and that is
www.macsales.com, because Mac upgrades is all they do. Also, are you sure you got the correct RAM? There are two 2.5Ghz G5s, and they both take different RAM. Do you have the two 2.5Ghz G5 CPU model that takes PC3200 RAM, or do you have the two dual core 2.5Ghz G5 CPU model that takes PC2-4200 RAM? To find out, click on the Apple menu and select
About This Mac, and then click the
More Info... button. In the window that pops up it will list some info about your G5. What you want is
Number Of Processors: and
Total Number Of Cores:. The two CPU model will have 2 processors and 2 cores, while the two dual CPU will have 2 processors and 4 cores.