The 8800GTX/Ultra are about 270mm long in the pcb, but that's an estimate from left over packaging. I don't have any of these cards at the moment to measure.
For reference:
The 8800GTS320/640 are about 230mm long in the pcb, plus another 10mm for 6-pin socket. (Don't have a GTS256/512 to measure).
The release notch on the PCI-E slot is around 145mm (5 3/4")from the back of the case, so scaling this to the photos, it is about 63-64mm, making the whole 9900GX2 close to 270mm, which is the same as the current GTX/Ultras.
8500GT is a short card, so you will need at least 270mm (10 3/4") from the blanks in the back of the case to your drive bays. You may want to allow another 12mm (1/2") for getting the card in/out of the board and case, and clearance for cabling.