be80be:
I had this same problem I got 6 atmega 328p- pu they all show up wrong id but there the same chip as in the uno r3 i got them to work.
But this Device signature = 0x00ff00 means you have messed up the chips fuses.I'm trying to figure if there is way to fix this cause I have one chip that that happen to too.
I can us the -f to read the chip and it shows the fuses are set wrong but I guess the arduino as isp can't set them now cause there wrong and have the chip not running from int osc. which is set on a blank chip.
Yes I can read the device signature = 0x00ff00 but sometimes I can read other values.