Flea collars aren't effective and can be harmful, Anneka. You're much better off using a spot-on flea treatment from your vet (we use Stronghold, but there are a couple of options so you can find out what suits your cats best).
Then, if you want to use a collar for ID purposes, the best ones are safety collars, like the
Kittycollar ones (there are some pretty ones in the auction).