I dont think they make "half size" video cards, if your talking about length. Just about any video card should fit in your case unless of course you dont have enough room and cant fit a "dual slot" card.
Unless your 6800GT is extra long, there is no reason why it will not fit if you have a standard Micro ATX mobo or case. Now, if your case has low-profile slots, then that is different altogether. If that's the case, I think the best low-profile card you can get is like the 7300GS...
Any regular video card will fit in the Qpack provided it isn't extra long... in fact, I see no reason why your 6800GT shouldn't fit... my dad's case is a Qpack and from a quick eyeball, even a 7900GT might be able to fit.