Re-installing disc images on Old Pcs

Hello,
I am responsible for the upkeep of approx 100 old ibm netvista Celeron 1.1s with 18 to 20 gig hard drives. they only have USB 1.1 and I am looking for the best way to reinstall a complete backup image to the hard drive. The image works through usb 1.1 and takes about 4 hours to complete. When I have to do 20 or so PCs at the same time I simply don't have enough external storage devices to have them all working at the same time. When I use usb 2.0 on some newer machines I can have them ready in 20 minutes so it is not a problem. The cd drives are also not DVD capable so I have a couple of options. 1. Is to temporarily connect a dvd capable drive or temporarily drop a usb 2.0 pci card in to get the job done and then remove the hardware at the end of the process and replace where applicable with the original hardware. Is there any other suggestions on how to speed up the re-imaging. The imaging is the best way to change considerably differently configured machines from 2 different centres which get moved from centre to centre when required. I've also argued with my employer that for all the time I spend keeping these old pcs working 24/7/365 surely new pcs would be a cheaper alternative. They disagree. Hmmmm. Help !!!