Exchange 2007 RPC requests

I have a issue with the clients saying it takes too long to open shared contacts etc. Server is a Poweredge 2900, windows 2003 x64, 8gb ram, mirrored drives. There are about 19 mailboxes and about 10 to 12 on at a time. They all have windows mobile or iphones syncing at the same time. The client computers all have vista and office 2007.

How do i go about finding where the bottleneck is?