If you monitor using perfmon over time, memory is the same as other counters. If you don't know how to create a log file using perfmon, then search on MSDN for perfmon log files to get the step by step.
In terms of which counters to monitor, it's more complex. I would watch target and total server memory from SQL Server to get an idea of whether SQL Server wants more memory. You could look at what's discussed in this article: http://msdn.microsoft.com/en-us/library/ms176018.aspx