SCSI HDD upgrade for Dell server. Please help !

I have a Dell PowerEdge 1850 server(in production). The two 160GB SCSI HDD’s are in a RAID 1 configuration.
The HDD has two partition 40 GB for system and 120 GB as a second partition. (I run Windows 2003 Server Standard)

The problem is that we are running out of space on the system drive and we are also low on the second partition.

I would like to purchase two new HDD’s but I have some concerns regarding this upgrade and about minimizing the downtime.

What would be the easiest way to image this system onto a new, bigger HDD ??
What tools would you recommend in order to do this procedure ???

Your help would be much apreciated !!!

Thank you.