Ya. You have plenty of room in there. The only thing you'd probably worry about is the heat sink there, but I doubt it'd be a problem.
Btw. The ruler doesn't help when it's closer to the camera due to perspective =P.
An HD 4850 is 9.25" long (not including the 6 pin connector that will be sticking out of the end of it).
The HD 4850 will squeeze in there, but good luck getting the 6 pin to fit.
Also, check to see if your PSU is powerful enough (about 20A on the 12v rail) and even has a 6 pin PCIe power connector.