Replacing a single failing SCSI drive that may be in RAID?

Hello,

Last year we ran into an issue with QuickBooks 2009 Enterprise having slow network performance and other issues. After posting here EE members helped us narrow it down to a hard drive being a possible cause. The manager elected not to replace the drive, because after the troubleshooting steps network and server performance returned to normal levels.

This week the problem has returned and the drive is making very audible noise. The current set up is as follows:

Dell PowerEdge tower
SCSI 2x Seagate 73GB 10K RPM 68 pin drives
It lists two separate drives (E.G. C:\  and F:\ where QB company file is located)
PCI Adaptec RAID controller
Windows 2003 SP2 Server OS
QuickBooks Enterprise 2009

Not sure which drive is failing, but would it be possible to replace a single drive in this setup or would two drives need to be purchased? It's a 2006 system and I believe the PSU may also be on the blink, at least the fan anyway. That being said, would a new server be more cost effective?

Thanks and hopefully I've been clear on the details. If not, I will try updating this more tonight.