The time lag on movies and youtube videos not the fault of the video card. It's caused by your internet connection.
I checked the specs on the Asus site. It just said "1333 MHz DDR3 RAM". The Crucial site:http://www.crucial.com/
has a system scanner that will give you a recommendation for compatible memory.
For a memory primer, check wikipedia:http://en.wikipedia.org/wiki/DDR3_SDRAM
An 850 watt PSU is massive overkill. One of my systems has an OC'd Q9550, 4 GB RAM, a GTX260 - a card with similar power requirements to yours, a Gigabyte EP45-UD3P motherboard, 3 hard drives and an optical, and a Soundblaster card all powered by a Corsair 750TX.
Running 3 instances of Prime95 to load the CPU and 3DMark06 to load the GPU, it pulls 375 watts from the wall as measured by my Kill-a-Watt meter. Figuring 80% PSU efficiency, the system pulls 300 watts from the PSU.
I would recommend a lower powered, higher quality 600 watt PSU.