Did you put that 7800 in yourself or did it come with it. I tried configuring one online myself and I couldn't find that option.
They changed the option to an ATI card.
Where do you get...
eBay seems to have a few.
From the pics on eBay, they look quite big, if your case only just have enough space for the HD, you might have to rethink. Maybe an external SATA enclosure? Icybox do some nice ones with lights