I purchased two memory modules "Kingston KVR1333D3S8S9 / 2G 2GB RAM (1333 MHz 204-pin, FBGA) DDR3-RAM" for my NAS QNAP TS869 PRO. There is a YouTube video showing the installation of the two memory modules in a QNAP TS869 PRO. The already included 1GB module can easily be removed and replaced with the Kingston module. Outside, in the actual expansion slot, is given to using the second module. Thus, one obtains a total of 4GB of memory in TS869 PRO. This has worked wonderfully for me. This should include the models TS469 PRO and TS669 PRO function, as these use the same hardware.
The YouTube video shows the for the for the QNAP TS659 PRO II. But it was the same for my QNAP TS869 PRO 100%.
The YouTube video, just search for "QNAP 659 Pro II - 4GB Memory Upgrade - YouTube".
Blank note things, precisely to acquire the said module: "Kingston KVR1333D3S8S9 / 2G"
In other modules with similar names (eg without 2G) there are problems, so report users in some forums.
Whether one is necessarily 4GB required course questionable. But for 24 euros (as of 06.12.2012) it can not hurt. Thus, perfect for my purpose and 5 star.