I think you have two big issues facing you. The amount of memory and a database that can use more than 1GB of RAM and 4GB of space. I as a solo doctor after four years on SQL Express still only have about 1.4GBs. But, with 20 doctors, well, multiply by 20. At some point, AC will have no option but to move to SQL 2008 Express which will give you 10GB of space. Even so, you will run out of space. Understand, that SQL is nice and gives you a bit of a cushion such as 4.4GBs.
It's too bad AC doesn't make it easier to make an instance with any SQL you want. Even SQL 2005 Standard will give you the OS maximum of RAM and no limit on database size. That would certainly make things comfortable. Of course, I would recommend, if possible, 2008 or 2012. The only advantage of Enterprise edition on 2005 is the number of CPUs and a few other things such as partitioning.
Given it can use the OS limit of RAM, you do have the option of setting the minimum and maximum amounts of RAM. If you did have 24GBs of RAM, consider using 4GBs to 6GBs of RAM for SQL.
When you see SQL taking up more and more RAM, this is not a "memory leak." This is simply SQL grabbing as much memory it can which correlates with pages which is basically remembering your most used queries. It is certainly better to have SQL delete the bottom MB of pages as it stores new than to have to reboot and loose the entire GB of memory. It would be nice to move to standard. ps: it can be done.
