Power Mac G5 not reading RAM

I just recently installed 2 1GB PNY DDR2 PC2-5300 RAM into my dual 2GHz processor Power Mac. It's not reading that I have any RAM installed. I currently have 2 1gb sticks and 2 256gb sticks, equaling 2.5 gbs of RAM right now. I tried taking the 256gb of ram out and replacing it, but that caused the computer to whir really loudly and not boot up. So, I put the old RAM back in and everything worked again, and so I tried putting the new RAM in another 2 slots. Now, the computer works fine, it just doesn't read that I've installed any new ram. I looked at the About This Mac>More Info>Diagnostics and it said this:

Power On Self-Test:

Last Run: 5/26/09 2:21 PM
Result: Failed
Failure type: Memory
Memory slots: DIMM4/J7100, DIMM5/J7200, DIMM6/J7300, DIMM7/J7400

So I'm not really sure what the problem is, or what I have to do to get the RAM to work, or if it's just because it's not compatible. It says on the package that it can work with 667mhz 553 mhz and 400 mhz, so I assumed it should work. Thanks,

The G5 towers used PC3200 DDR or PC4200 DDR RAm, depending on the model

so even though it says its compatible, it has to be PC4200? I'm currently running DDR2 SDRAM PC2 4200, so that means the PC2 5300 won't work despite it saying it's compatible with PC2-4200?