Memory usage in all Windows Vista+ versions is setup to keep as much stuff in memory as possible. This is the most effective way to do things as it prevents reads from disk, and flushing unneeded pages from RAM takes significantly less time than filling those pages from disk. The same problem you're seeing applies to MSSQL servers, Exchange, anything else that has a use for immense amounts of RAM since they will load as much into RAM as possible at any point.
If you're not seeing actual issues with paging out operations (Disk thrashing), Out of Memory errors in other applications, or performance issues with other applications, you can safely ignore high memory use. Empty RAM is not-needed RAM. Conversely, if you're expecting high memory usage on a machine and you're seeing zero-ish usage, there's likely an issue.
As for monitoring of the memory usage, if you're monitoring only total RAM used, I would stop getting alerted at high levels. Keep alerting on for low RAM utilization the same as you would for low CPU utilization on a server which has applications that keep CPU at 20% (or whatever) when idle. If you're monitoring what application is using what amount of memory, and the server is hosting multiple applications, you may want to test your other applications without MSMQ running so you can get an accurate representation of how much RAM they'll use if given the opportunity, and adjust allocation and monitoring to account for those needs.
At our organization we've stopped monitoring high RAM consumption on all servers which we expect high RAM utilization; it does nothing but trigger alerts that need to be silenced, and obfuscates the legitimate alerts.
This article has some high level info about how memory management was changed in Vista, there have not been hugely significant changes to the memory manager since then, AFAIK.
