Q: Reuse old 160GB drive from G5?

You need an external enclosure for the drive. It must support SATA and have a USB port to connect to the computer. You should find a suitable enclosure at OWC as well as many other component vendors like NewEgg, Best Buy, Buy.Com, etc. You will need an enclosure for a 3.5" HDD.

Assuming that the G5 died from something unrelated to the hard drive,(i.e the hard drive is still fully functional) all you would need is an external USB enclosure to turn the hard drive into an external Usb drive.

Probably the cheapest way is one of these...

http://www.ebay.com/bhp/usb-to-ide-sata

I'd recommend one with AC power brick though.

You'd better get auto off capability enclosure which is automatically power off when system is going down.

http://www.amazon.com/gp/product/B00APP6694?psc=1&redirect=true&ref_=oh_aui_deta ilpage_o01_s00

I have one of these. Works well.