If 720 comes end of 2012, the I expect an GC -> Xbox gap
(no real discernable difference)
if 720/PS4 comes end of 2013, then I expect a PS2 -> Xbox gap
(noticeable but nothing that shouldn't easily scale between the 2 groups)
if PS4 comes end of 2014, then I expect a DC -> PS2 -> Wii gap
(engines should scale, but graphical leap will be easily recognized)
Truth is that graphically, regardless of what 720/PS4 have packed under the hood, all 3 next-gen systems should be able to support the same graphical effects on the same exact engines.
It will all come down to which machine can do more of them at once, across the whole scene and at 60fps when pushed, and which ones need to be scaled back a little to get a similar effect. Outside of the detail oriented internet folk that will compare everything side-by-side, I don't expect the average Joe to see much of a difference (graphically) between the 3.
Feature set, controller, User Interface and exclusive games will be the difference maker next gen, not graphics.