Tiling/Floor Experts needed

[Coley]

Ok guys, I'm looking for some experts in flooring and tiling, here is my situation and what I currently have:
-5/8" T&G plywood (basement) subfloor sitting on 2x4's (on the flat) spaced at 24" o/c on a concrete slab.
Now, I want to put down not only tile on this, but a heated floor within that.
So I would like to know about the 'next layer' on top of this 5/8" plywood prior to tiling and heating pad/floor stuff in order to avoid floor cracking etc but also to promote a heated floor?
-ie:.....1/4"? plywood to thicken up the existing floor rigidity?
(note: I would be screwing all of this stuff down probably at 6" intervals...not nailing)
or....3/8" plywood to bring the floor up to a full 1" thick?
or....1/4" cement backer board
or some other surface layer product before the tile...or?
Anyone who has some good, seasoned and successful insight into this....I'm all ears.
Again Many Thanks!
Coley