What hard drives can I use as replacements?

If the server is under warranty then it will be best to call dell, else if you r aware with details of harddisk u can purchase from any store. If u are not aware of how to install it by self, best call dell or service engineer. the capacity the server can support is large you can go for 500GB harddisks

Call Dell with your Service Tag.
Lifetime support is included on all Dell servers.
Last time I called them I dialed (800) 945-3355
They can sell you the hardware you need and guarantee it will work with your server. They might even be able to help explain the process of installing the drives.

I recommend that you use Acronis' Imaging software (www.acronis.com) so that you can make an exact copy of your data and apply it to the replacement volumes. If you are using a RAID1 array make sure the new array is completely built before applying your image. Builds can take a long time. I created a RAID1 array with 2-320GB disks and it took 3.5 hours to build the RAID1 array.

I hope this helps.

Please vote for my solution if it helps.