Source: Microsoft Technet
“Memory is often the source of performance problems, and you should always rule out memory problems before examining other areas of the system. Systems use both physical and virtual memory. To rule out memory problems with a system, you should configure application performance, memory usage, and data throughput settings, and then monitor the server’s memory usage to check for problems. Application performance and memory usage settings determine how system resources are allocated. In most cases you want to give the operating system and background applications the lion’s share of resources. This is especially true for Active Directory, file, print, and network and communications servers. On the other hand, for application, database, and streaming media servers, you’ll want to give the programs the server is running the most resources.
Here’s an overview of counters that you’ll want to track to uncover memory, caching, and virtual memory (paging) bottlenecks.”
Read more on source.