I would recommend measuring Avg Disk Sec/Read, Avg Disk Sec/Write, and Avg Disk Queue Length within performance monitor. You can use SQLH2 to help capture this information over a period of time. I would also suggest using IOMeter or SQLIOSim to benchmark performance.

For more information -

http://www.sql-server-performance.com/tips/monitor_io_counters_p1.aspx

http://www.sqlteam.com/article/benchmarking-disk-io-performance-size-matters