Backup bottlenecks

What is the best way to figure out what the bottle neck in a backup would be?

I have a DELL 124T attached to a DELL PowerEdge 2950 via Ultra 160

Inside the 2950 I am running ESX 3.5 and a Server 2003 Virtual server. That is connected to our network Via Intel 4 Port NIC that connects to a DELL 6224 Switch.

The data we are backing up is coming through a Dell Poweredge 2850 that is hooked to an AX150i SAN via iSCSI.

The problem is we are only getting about 380 MB per minute of backup speed and we are backing up 1.7 TB of data on a full backup.

Any ideas?