Microsoft gives you a lot of good information here: http://technet.microsoft.com/en-us/library/bb125079.aspx
Off the top of my head, I believe you want 32KB stripes and you'll want the partitions aligned to the stripe boundaries. Don't take my word for it-- read the guide. I can be an idiot sometimes. (smile)
Edit: I said 64KB first. It should be 32KB. See-- I'm an idiot.
Also see: http://blogs.technet.com/evand/archive/2004/10/14/242127.aspx
An aside: Reading Evan Dodds posting again made me recall that I did see an adverse performance situation on an Exchange 2003 installation where the SMTP queues were put onto the same spindle as the transaction logs. I recall you saying that this box is going to be a back-end server, but it will, obviously, still be receiving incoming email from your front-end servers into SMTP queues. The situation I recall is much the same as Evan Dodds described-- the SMTP queues were on the transaction log spindle and the spindle ended up thrashing between trying to do sequential writes of the transation logs and random writes on the SMTP queues. Moving the SMTP queues to a spindle of their own ended up making the box a lot happier. (It got the disk queue length way down on the transaction log disks, and decreased the user-visible symptom-- RPC timeout popups galore!) Watch out for that.