What's important is how much memory you are using in your buffer cache. SQL will use other memory for other non-buffer memory pools. If you are using SQL Express, then your buffer pool is limited to 1 GB. Unless you change the max settings on SQL, it will eventually take as much memory as it can get. SQL basically things that the OS is only for it.
The memory will be used to store pages, which are about 8 KB in size. These pages will be former queries made. (not the actual returned data), but the path it took to get the data. All the tables it went to, etc. Some paths or queries aren't efficient, so storing queries which may be used again it helpful. There is a reason SQL has a buffer pool. And, there is a reason it grabs memory. With five computers, I simply don't see why you would need SQL Standard or Enterprise. First, it is rather expensive. And, second, you would need help with the install. It sounds like Indy would be willing to help you. Certainly, offices with a much larger amount of users can benefit from using Standard. Of course, a standard version would have to be limited to the maximum memory or it would take all of the OS's memory. Yes, you can simply reboot the server to release all the RAM. Or you can type services.msc in the run box and Restart the AmazingCharts SQL instance either by right-clicking on it or using the choice at the top left. You can also run a script to do this. But, this is a little akin to your using your computer and running six apps perfectly well on the amount of RAM you have, then deciding to logg off to free all the RAM. Now you log back on, and great, you have all your RAM back, but all of your apps are gone. Just an analogy. So, you restart SQL, and it releases all the RAM it took, and releases all the pages as well. So, you lose the benefit of stored queries in the pages, therefore, every query you make will have to hit the database on your hard drive, decreasing performance. What can happen in an office that is making a lot more queries is that while SQL is dropping the oldest and dirtiest pages, it may not be able to keep up and have slower performance. Standard and Enterprise editions will allow a lot more RAM, are more efficient and will have more time to delete dirty pages. I will defer to Indy on the fact that he has solved this problem with full versions of SQL, and if that were something you wanted to do, trying a restarted SQL would be the first step. If this solved your problem, it would be worth it, but SQL Server Standard 2012 isn't cheap depending on the licensing. It would run you at least $700.
It would also be more helpful to use Process Explorer and Perfmon (found on your computer) as they give tons more information, and they are more accurate.
I still don't know the amount of RAM memory for the OS you have on your main computer.
You can see my PM for further info.
