Upgrade Disks RAID-5 System

I recently began supporting an office with a Dell Server. It has W2k Server 2.8 Xeon 1.5GB Ram.
It also has 4 Hard Drives in a RAID 5 config.
They are Seagate Cheetah 37GB SCSI drives attached to a PERC 4/SC Controller. Logically there are 2 drives, C: 12GB and D: 90GB.
I am not all that familiar with RAID 5. Therefore I’m not positive what I’m looking at.
The big issue is this, the company wants to upgrade them to 73GB drives because of space constraints.
I’m not sure where to begin.
The server is in production 24/7.
Any suggestions?
Thanks, GP