It seems that the Hot Fix driver should simply be called the "Increase your 3DMark score" driver. Apart from the two synthetic benchmarks, the only game to really see an increase in performance was Unreal Tournament 3. Outside of that, most games had a negative affect when adding more cards.
Hopefully this is something that AMD attack over the coming months. With more and more boards offering two or three PCI Express x16 slots, placing a third card into your system at these prices could be more than doable.
Two cards really is the sweet spot for HD 4850s at the moment; we see some good gains over a single card and the cost is still relatively low compared to the competition. Now all we have to do is wait for the HD 4870 so we can see what that can do for us.
AMD have a winner on their hands with the HD 4850; just don't expect to see the same value for money as you start climbing the ranks with three or four cards. It could still be a while before we see these kinds of setups represent any form of value for money, much like NVIDIA with their Tri SLI GTX 280 setup.