No the normal BTX (similar to mid tower atx) cases will support atx psu's. And your worries are to be put on hold, I found this in the link I posted-
"For those of you heavily invested in your ATX motherboard and AGP graphics card, there's no reason for panic. It will take quite a while before the death of ATX; and although we've done a lot to get rid of the "beige box" with the latest generation of ATX cases and the advent of small form factor systems, the final nail in the coffin of boring computers will be driven by BTX."