Basically, as I know, its that BTX is a modified form of ATX to have better airflow, the whole turn it upside down thing CAN be done ATX boards, although it is designed for BTX. BTX places the processor at the front of the case and sticks the GPU on that same level to cause an airflow that throws out the GPU air and the processor air. There is the Dell BTX which again, dell being dell have modified it, so its not true BTX but its pretty close. If you want to see what its like, look at the inside of the Dell Dimension 5000s or have a look at JoshD's case in the case forum. They are quite good, but are a pain up the arse to do wire management on!
Ruan