Seeing as the 4850 performs ok at 1900x1200, I'm not sure you need anything more then a single 4850 for 1600x1200. Unless you crank up the AA, I'd just get the 4850. You could go with the 4870, but I wouldn't bother. Save the dough now, upgrade again in a couple of years when the 4850 isn't doing so well anymore.
if you have the money, get the 4870, if you are on a little tighter budget, get the 4850. for your monitor resolution the 4850 will run perfectly (that is what i have and i run the same resolution), but if you have the budget the 4870 will boost your performance even more