Ideally, you should try to find a drive that is identical. If you can't, it will still work with another drive. The actual RAID implementation will continue to use the same disk space on both disks. If it's on a server, I am sure you are doing regular backups.
Make sure run a complete backup of your server prior to changing any hardware, especially a hard disk. Also, it might be a good time to test that your backups can easily be restored. I have heard of smaller businesses thinking that they were running a successful backup, to find out that were not able to run a restore from backup. Very embarrassing for them.
Hope that helps,