Killer Sudoku - Stumped

[pistolero]

I've been working on this week's "Outrageous" puzzle off and on throughout the day and now I'm stuck. Can anyone give me a clue here as to how to proceed?

[durron597]

Ok, I figured out one step, I may look again later if you're stuck but this is progress.

In white:

<font color="white">Look at the 78 box in the lower left corner. One of those two values is impossible with only a small number of layers of nested logic.</font>

[mute]

Since, I can't figure out Durrons clue (maybe you've already figured this out, though), here's something else that might help:

<font color="white">You can exclude 4 as a possibility in the top center quadrants bottom right corner (where the values are "1457"). If this was a 4, then there would have to be a 4 in the top right corner of the middle right quadrant. This would make it impossible for any 4 to be in the top right quadrant</font>