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 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.
