your main problem will be that the upper bay is not ventilated enough to run a hard disk. It will overheat. how much is dependant on how much you use the disk.
you could get a fan to fit in the top bay, and cool it that way, but I think you're going to buy a 2nd IDE card, a big hard disk, and a fan t cool it with, you're going to turn your 6500 into a money pit real quick.