This week I saw an issue with an AOS which was consuming too much memory (~1.8gb) and then throwing users off.

Put simply this was because in the server configuration utility, database tuning tab, the maximum buffer size was set too high. In my experience this is a commnon mistake. The field is actually held in kilobytes (although it doesn't say that) not bytes! So adding a value like 30000 means that almost 30mb will be allocated for each operation and the AOS will very quickly consume all available memory.

1.8gb is the absolute maximum that the AOS can consume before it gets into trouble regardless of how much memory is in the machine or use of the /3gb switch. This is because it's a 32 bit application.

In AX2009 there is a 64bit version of the AOS which means that 1.8gb is no longer a limitation, the limit will be the amount of memory available in the machine. However - if memory usage is above 1gb then you probably have an issue.