Old eMachines T5048 has one 512MB PC-4200 DIMM and one 1GB PC-5300 DIMM (PNY). I swapped in another 1GB PC-5300 (Rendition/Crucial) but the BIOS doesn't see both DIMMs. The Rendition DIMM works by itself but not with either of the other DIMMs. Will flashing the BIOS fix this?
Flashing your BIOS unfortunately will not fix the problem.
You were using two different speeds of memory and with different specifications/voltages. Try getting the exact duplicate of the first 1GB stick. If that doesn't work, and if you know how to get into your BIOS and it has a memory tester, use it. You might have fried the memory controller...
Does your machine boot at all? Can you access the BIOS?
Thanks to all for your responses. I got another 1GB stick and that worked with the other new stick. My guess is that the original 1GB stick runs happily clocked down to match the PC-4200 stick, but does not work at full speed when matched with another PC-5300 stick.