When you configure a Virtual Disk (array) on the PERC, the OS sees that array simply as a "disk", as though you had inserted a regular/single hard drive into a desktop.  So, Windows should just see the drive (in Disk Management - you will need to initialize and format it before it is seen in My Computer/Windows Explorer). The RAID controller is capable of managing up to 64 separate Virtual Disks on the same controller, so managing a 3-disk RAID 5 and a 1-disk RAID 0 will be a piece of cake for the controller.  It is possible to add your 500GB disk to your RAID 5, but it won't happen without you explicitly telling it to do that - in that case, only 250GB of the disk's 500GB would be used, growing your 500GB RAID 5 to 750GB.  If you configure the disk as a RAID 0 (your only option with a single disk), then it will appear to the OS as a new "disk".
I've got a Dell Poweredge T610 server with 3 x 146gb Seagate hardisks on controller 1.  We're running short on disk space and was wondering if there was anyway of installing a new 500gb hard disk away from the raid as a standalone drive. This will be used purely for music/audio files which I don't want to be included in the raid. The server currently runs on Server 2003 x64 with 16gb RAM.
Installing a standalone HDD