Geforce 9800 GX2 will have a massive cooler in-between two PCBs. The cooler itself will have a two copper parts that will fit on the GPUs and it will be placed in the middle.
The board needs two power connectors, one 3x2 and one 4x2, and it will need about 200 Watts. It will run very hot, but we are sure it will be stable.
In the end the performance matters, and this will be fastest thing around.