My sister has 2011 iMac currently with 4GB memory. She would like to have more and I was wondering if I could salvage a few years old computer for memory. I would like to know if such old RAM would even work with 2011 iMac. System Information software gives following information about the memory that old PC has installed (I don't know what is relevant so I'm just going to include everything): Memory Type DDR2 [PC2-6400] Manufacturer Kingston Model 99U5429-007.A00LF Serial Number 162A8131 Capacity 2048 MBytes Speed 400 MHz (400 x 2 = 800) Manufacturing Date 2008, Week 27 EPP SPD Support No XMP SPD Support No Supported Frequencies 266 MHz, 333 MHz, 400 MHz PC has two of these units so with both of these iMac could be upgraded to 8 GB total I did some searching and found out that 2011 iMacs should be installed with 1333Mhz RAM. If I understood correctly the memory in question is 800Mhz. I didn't find any information if this kind of memory is supported or not. So could I install this old memory to my sisters iMac? I know that memory is quite cheap these days but we are really looking for a budget solution here. Would this slower older memory have any negative effects on iMacs performance compared to just using the 4GB currently installed? Would the machine be significantly faster if I used 1333MHz memory?