Adding RAM is rather simple. I would check your documents as with four slots, you may need to do 1 and 3 and 2 and 4 or 1 and 2 and 3 and 4. All of them have calculators which allow you to enter the data to get the right RAM. For me, in a regular computer, RAM is RAM. This is probably overkill, but you can download Memtest + or Memtest 86+, burn it to CD and test the RAM. You just have to have it seated all the way down.
If you were only using 4GB, that is likely your problem. My guess is you are using a 32-bit computer meaning you were getting 2.7GB (probably more like 3 depending on your reserved address space) of RAM. SQL will then take 1GB (as it should), and you are down to 1.7GB of RAM. The computer takes the rest. Once you get down to below 500MB or so, the ENTIRE computer becomes slow. Again, you focus on AC and it's the computer. Add the extra 4GB and you will likely be fixed.
Yes, anything over 4GB of RAM is wasted except you may be able to use it in a 64-bit OS. Not all computers can handle 64-bit OS.
Again: 4GB lose at least a GB, down to 3GB, use 1GB of RAM in SQL and you are down below 2GB. That has to run all the services, programs, etc. It isn't long before you are running less than a GB of RAM, which really isn't enough to run a computer adequately. Hence, people reboot, spilling all the RAM back to the computer, and they are back up to 1.8GBs or so. But, that comes as a huge price as you lose all your pages (queries)which were stored in SQL). Not good.
Now with a 64-bit OS, you can handle 16.8 million terabytes of RAM, but that is like driving a Ferrari on a 35 mile back road.
Check your page file and see if it is set to 6GBs. That may be of some benefit.
