Supported is DDR3 PC3-10600, DDR3 PC3-8500, DDR3 (non-ECC). However, the BIOS will only allow DDR3-1066/PC3-8500 according to Acer; see -> http://us.acer.com/ac/en/US/content/model/LX.R4402.263
Crucial - http://www.crucial.com/store/listparts.aspx?model=Aspire%205552&Cat=RAM
To know, boot into the BIOS and see if DRAM/Memory Frequency options are available; otherwise there's no point in getting anything faster. Also, DDR3 1066 CAS 7 is slightly faster than DDR3 1333 CAS 9; think of Frequency as speed but Higher CAS and l-o-n-g-e-r stops. In any case the difference is 1%~3% tops and unnoticeable.