I don't think its worth buying RW discs for recovery discs. Just use DVD-R discs for the greatest compatability (DVD+R should be ok as well). Single-layer should be fine (4.7GB) but check with your recovery disc creator as it may need a Dual-Layer disc. (Mine did)
Also, I think most PC's can boot from usb-keydrives nowadays, so you may like to make a recovery keydrive as well. They are fairly cheap now.