Are your SAN drives performing well?
Those that know me know that I consider SAN's to be one of the major bottlenecks to high performance SQL Server databases; certainly my experience over the years has often shown this to be the case - however that's a whole other area for discusion, what I though I'd draw attention to is a superb post from Linchi Shea
http://sqlblog.com/blogs/linchi_shea/archive/2007/09/18/sql-server-and-sans-the-queuedepth-setting-of-a-host-bus-adapter-hba.aspx
For my sins I am actually SNIA certified, www.snia.org, in fibre channel systems and I was aware of how HBA's often needed to be reconfigured with busy SQL Server databases, however I didn't have any reference document to draw upon - now I do.
I can confirm that I watched a colleague test this and the results were interesting, sadly I can't publish details but I would advise that if you have a SAN attached drive then monitor the io completion times for a set query, alter the queue depth and re-run the query and monitor again. In our case indications were that a queue depth of 128 gave the best performance. Test was on x64.