It really doesn't matter. If the RAM can pass an overnight run of Memtest or Windows Memory Diagnostic, it will probably not fail. IF you are overclocking, you do need to look more at RAM features and speeds. No wonder you are having issues, it is difficult to find detailed product information at HP's site on this model. However, the deactivated product page
at TigerDirect indicates the All-in-One follows the industry standard for those types of PC's: A laptop motherboard glued to the back of a monitor. That said, DDR3-1333 should work so long as it is laptop memory. I believe the Crucial 8GB (2 x 4GB) 204-Pin DDR3 SO-DIMM DDR3 1333
would work. All SO-DIMM DDR3 models at Newegg's site are 204 pin, and that is DDR3-1333 PC3-10600 memory.