Server 2008 Cached Memory in Task Manager Explanation?

Hi,
I'm looking for an explanation on what the Cached Memory display in Server 2008 Task Manager is all about, and if it's configurable.  I have many Server 2008 servers, from Hyper-V servers to File Servers, Exchange Mailbox Servers and SQL 2008 Servers.  We have noticed on several systems performance issues that seem to be memory related, where the Free Memory on the box will decrease to near 0 while the cached memory is a very high figure.  I don't want to get caught up in troubleshooting each application type as that's not the issue here, I just want to know if there is a way to see what exactly is in this cache memory, and if it's configurable.  As far as I can see so far giving it free reign is causing issues.

Attached is a screenshot from a file server with about 12 TB or so of data, configured as a MSCS failover cluster.  As you can see from task manager there's no processes running using more than 250MB of RAM and yet my 14GB of physical memory is almost completely used by cache (12 GB).  This same file server running on 2003 server had 6 GB of RAM and ran like a champ, I thought I would double it when I migrated to 2008 and yet it's still all gone and the box feels sluggish.

The same scenario has played out on all my 2008 servers - I'm trying to calculate how much memory to give different servers/roles and this cached memory seems to be a complete variable.   I wouldn't mind so much if it just used all memory and gave it back to processes as they needed them, but that does not seem to be the case.  It seems once it's used process actually needing physical memory can only pull out of the Free pool, and performance is impacted.  Googling has found a few articles of people troubleshooting specific application types (like turning off synthetic NICs in Hyper-V etc) which is not what I'm looking for.

Is there a way to see what is using the cache?  Are there any configurable settings to change the priority of memory caching, what % it uses to cache, and how long it caches data for?  If I could change it I can then perfect performance for each of my server roles through trial and error.

thanks,

Pete Lill