The 1.6 single G5 uses PC 2700 RAM instead of PC 3200. But the 3200 should work fine in the 1.6, it will just run at PC 2700 speeds (unless someone else here has experience that the 1.6 is picky and won't run 3200's).
Here are the full specs for your 1.6:
http://www.everymac.com/systems/apple/powermac_g5/specs/powermac_g5_1.6.html
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The other RAM should also be PC 3200. It's not likely the previous owner had a nice machine and was running mismatched RAM that is slower than the other RAM. You can normally find a part number written on the RAM itself, you can always Google it and normally figure out what it is. In this case, you could also put it into the 1.6 to find out, (click on System Profile - Memory) assuming it can safely run PC 3200 RAM. Just keep the pairs together, they should be matched pairs.
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Crucial claims the 1.6 can run either PC 2700 or PC 3200...
http://www.crucial.com/upgrade/Apple-memory/Power+Mac/Power+Mac+G5+%28Single+1.6GHz+DDR%29-upgrades.html
"Each memory slot can hold DDR PC3200, DDR PC2700 with a maximum of 1024MB per slot."
They also only offer PC 3200 for that system right now, so it should be fine. I don't know if you will encounter problems running two different speeds of RAM, assuming the other 2 slots have PC 2700 in them, some Macs can be very finicky. If you have any Kernal Panics or odd problems with your 1.6 I would pull the PC 2700 out and put some of the other PC 3200 from the dead Mac in and see what happens.
You should also run Memtest on all of the RAM, swapping out as needed, to make sure there are no issues with any of the sticks. Instructions and download are here:
http://osxdaily.com/2011/05/03/memtest-mac-ram-test/
