Check the throughput of the SAN system as well as configuration of HBA queue depth.
See the virtual file stats if there are high IO read and Write stall( sys.dm_virtual_file_stats).
Capture IO, Memory and CPU counters
Check Memory Related Perfmon Counters
SQLServer: Memory Manager\Memory Grants Pending (Consistent higher value than 0-2)
SQLServer: Memory Manager\Memory Grants Outstanding (Consistent higher value, Benchmark)
SQLServer: Buffer Manager\Buffer Hit Cache Ratio (Higher is better, greater than 90% for usually smooth running system)
SQLServer: Buffer Manager\Page Life Expectancy (Consistent lower value than 300 seconds)
Memory: Available Mbytes (Information only)
Memory: Page Faults/sec (Benchmark only)
Memory: Pages/sec (Benchmark only)
Checking Disk Related Perfmon Counters
Average Disk sec/Read (Consistent higher value than 4-8 millisecond is not good)
Average Disk sec/Write (Consistent higher value than 4-8 millisecond is not good)
Average Disk Read/Write Queue Length (Consistent higher value than benchmark is not good)
MCTS,MCITP Sql server 2008.