Can the drives on them read DVD-RWs? What about dual layer DVDs?
Depends on what that sticker says, assuming the drive has not been replaced. It's hard to say with eMAcs; becasue they were an edu-model, they probably have more optical drive posibilities than any other Mac. Some even shipped with NO optical drive, presumably to prevent students from bring software from home.
The MacTracker database (free from http://mactracker.ca/) lists the following possibilities for optical drives in a 1.25G eMac:
None, CD-ROM, DVD/CD-RW Combo, or 8x SuperDrive
Also more tech info here:
http://www.everymac.com/systems/apple/emac/stats/emac_1.25.html
The combo drive reads DVDs. Reading DVD=RW may depend on where they were burned and the media brand. The next model, the 1.42Ghz, shows the SuperDrive as dual layer; no such entry for the 1.25G.
If you are thinking about burning a system disk from another computer, be aware that almost never works on Macs.