Sizing and configuring the hardware for Exchange server is dependent on many factors: How many mailboxes will the server host? Will you be hosting handheld synchronization of any kind? Will the clients be using MAPI, POP, IMAP, or RPC over HTTP?, How many users will be connected via OWA simultaneously?, How many IOPS per mailbox, etc., etc.
At the very least you want to separate the Exchange log and database files onto separate physical disks (RAID arrays). You'll get the best overall performance if you use RAID1 or RAID10 arrays.
Creating a single physical array and creating multiple logical volumes on that array gives you logical separation but not physical separation as the logical volumes will be contending for the same underlying physical disk. If you mix I/O access types (Sequential vs. Random) on the same physical disk you're asking for performance problems. My suggestion is not to go that route.
Without knowing more details of your environment here's my suggestion:
Create 3 RAID1 arrays. Use the first RAID1 array for the OS, the Exchange binaries, the swap file, the Exchange working directory, and the SMTP directories. Use the second RAID1 array for the transaction logs. Use the third RAID1 array for the database files.
Some great information on optimizing your storage for Exchange Server can be found here: