While it's still unclear what real world benefits DX10.1 offers over DX10, Physx on Nvidia GPU's allows the GPU to essentially do what the Ageia Physx Processor was designed to do, only better.
Physx allows for destructable environments, create higher levels of immersion into a Physx enabled game. E.G. shoot at a wall and watch it crumble or blow a hole in it. Have rubble that is dynamic
as a result. Realistic weather effects such as being pushed by wind, hail falling and bouncing off the pavement, tornadoes that rip buildings apart and pick up and whip around debris. It's kind of unlimited what can be done.
I have been messing around with Unreal Tournament 3 with the Physx package, and it is pretty cool.
And yes, it looks like a 9800GTX+ would fit in that case. Especially since the hard drive mounting bays are sideways. I like those kinds of cases. Makes it so much easier to remove/add drives when you need to.