Memory Help? What is the difference between Low Profile DIMM Memory and others and how would I know the difference if I had to buy some? My PC wont go low.

I have an Intel processor DQ965CO motherboard with the following specs and slots available. My goal is to identify the limitations of motherboard and figure out exactly how much more usage I can get out of this machine. Wanting to find out the best memory for it the best brands of memory if anyone has an Opinion. Finally what is the most powerful most performing memory I can put on the board for the best performance.
On this question about the Intel Legacy Board DQ 965 CO The machine has Vista for Biz but will soon have a new license of Win 7 Pro. I did study the compatibility documents on the link above.

1. This machine only shows a possible 8 GB of memory I wonder if it can take more and actually use it?
2. Under Compatibility/ Memory - I did pick up one thing about “low-profile DIMMs” So I looked it up and it says something about the speed of “low-profile DIMMs” So I can assume that I need to avoid those. However. How would I ever know if I was buyng a “low-profile DIMMs” vs how to know to get a High or Medium DIMM. sounds like Gasoline Octane..?
3. Wondering what is the best performing memory I can install for the board.
4. I have always like Crucial and Kingston. Are their any as good as but reasonalble costs. New or used?

I am including the images of some of the specs currently on the machine.