Fury's right, 12.4" is just too big, who cares how well it may be able to perform for hours on end without a break if you can't fit it in the box to start with.
Saying that thats quite a beast, and although i'm not a huge ATI fan anymore would love to see the benchmarks for that thing. However it would require not only a redesign of your PC/case, it may actually involve moving house to a large warehouse type flat to fit it in.
i quite like ATI but i'd prefer an Nvidia card to my crappy Radion 9000 pro but that card is quite insane i can imagine one of the dudes a ATI wakin up one mornin and sayin to himself "f*** this s*** i shall create a monster and i shall name him franken.... no i shall name him R600XTX muhahahaha!"