Are your disks just holding data, or are they the system/boot disks?
If they are just data disks, then I would recommend backing up the data to an external device and then restoring it to the new disks.
If they are system/boot disks then it depends to some degree as to whether you are running hardware RAID or software RAID.
Basically, you are going to need to boot to WinPE, and you are going to need to provide WinPE with the required drivers to allow WinPE to access the RAID array and read the data on the HDDs.
I'm not personally familiar with the Poweredge 1950 model's internal hardware architecture, hence I can only generalise on the RAID system.