I'm afraid I'll have to disagree with Indy on his point 2:
SQL Server Express 2012 uses only 1G of memory regardless of whether it is 32 bit or 64 bit.
Please note that maximum database size goes up to 10G from previous versions so practices that are filling up SQL Server 2005 Express have at least one reprieve when upgrading.
https://msdn.microsoft.com/en-us/library/cc645993(v=SQL.110).aspx
JamesNT
James there is no right or wrong. For all practical points and purposes, I would say just keep it simple and go with what you are saying. The reason express is free is the memory limitation to the buffer pool. This is where all of the pages are kept. There are many other pools that can utilize more memory, but I am not sure the user that only needs Express will notice the difference.
As far as needing something with more memory for the buffer pool and having to go to a standard version, I don't think a practice with less than 10 or so users would know the difference. The other issue with going to a standard version would be support. You could have any issue that has nothing to do with SQL, and as soon as support noted you had the standard version (if they could tell), your support contract would be terminated.
I think what AC could use for practices over 10 or more full-time users would be for AC to have people like Indy, you or Raja to install the SQL standard, knowing it would be done correctly and having all of the info they may need to do it that way. I mean that is the beauty of SQL (and other databases). You can increase their capacity meaning better and faster performance.
