You *should* be able to cram the 6800 GO Ultra in. But you *might* have some issues with the power supply. The 9300 was basically the XPS of it's generation, with a few less options base. I've gt a 6800GO in mine, and it still works just fine for playing DOD, CS:S, The Sims 2, ect.
The 7800 probably will fit, but I don't know if the power supply will be up to snuff to handle it.