I wanted to know if I could use a normal PC hard drive on my imac since I am running out of space and i need more room free on my computer.
Is it possible to use an extra pc hard drive on my G5?
If not what are the pricing range between the special apple hard drives?
You certainly can use a PC hard drive in or on a Mac as long as it has the same connector. There is no room to add a second internally, but if you are looking to replace the internal hard drive, check this out: http://docs.info.apple.com/article.html?artnum=300205 for instructions.
The G5 iMac uses a SATA hard drive (Serial ATA), so an IDE hard drive won't be compatible. Most PC's use SATA now, but if you are pulling the hard drive from an older PC, verify that it is an SATA hard drive. If not, just make sure you buy a SATA drive.
Another option would be to buy a 3.5" or 2.5" enclosure (FireWire or USB 2) and put the new drive in this. Then you can hook the drive externally to your Mac for added storage.
As long as you get a USB 2.0 hard drive or a FireWire hard drive, you'll be set.
The first thing you'll want to do when connecting the new external hard drive is to re-format the drive in HFS+ format (Mac OS Extended), as most external drives come formatted as FAT32 (which is good for cross-platform use of the drive, but if you intend only to use the drive with the Mac, format as HFS+). You can use Disk Utility located in the /Applications/Utilities folder to format the drive.
Sounds like an external drive is best for your needs, and you can get huge ones pretty cheaply these days. Best bet is to use the external for storage but keep your system files on the internal bus drive which is faster. SATA is 1500 mbps vs 480 (USB2.0) or 400/800 (FireWire) which you'll get external.