As stated above, get PC3200 DDR400 RAM for your G5. I think the CL of Mac BTO RAM is 2.5. You have to get a pair, one for each bank.
Do a search in the forum, and you will find lots of threads concerning this, including opinions and test of which work and which to avoid. You'll find that this RAM is a lot cheaper than the Apple RAM you bought. Apple tends to overprice their RAM. You can get some good deals out there.