If the Williams Sisters can remain healthy, I predict that the Justine/Kim era will be noted as just a "bubble" while the best players were out, then the Venus/Serena era will continue, with ocassional "upsets" mostly by Justine.
These three players are CLEARLY above the rest, with Kim and Jennifer next after a moderate gap. Jennifer matches up great with Serena, but has given up some of the mental toughness she had in 2001-2002.
Justine is a top quality player with ALL the shots one needs. She is clearly better on clay, with Serena close behind. She is getting better on hard courts, and WILL be a factor.
Kim is mostly a counter puncher vs the other top players, but could be great if she develops a more offensive game.