How to install OS on new internal?

Hello, Im a bit of a novice here. I recently replaced my hard drive in my Power Mac G5. The old hard drive died 2 weeks after the Apple Care ended. I purchased a new hard drive (Western Digital SATA 500gb) the physical install was very easy and clean. I started up the computer with the install disc in the drive while holding down the "c" key and was able to get to disk utility. I partitioned the drive and then set about installing the OS X onto the drive. In the installer the HD icon is unselectable, it has the red exclamation point over it. Really not sure what else to do. Searched around the internet for a while trying to find a solution, with no luck. Any ideas? Thanks for your help...