well, if your gonna xfire, you should do it with another 4850, dosen't matter what brand
if you pair your 4850 with a 4870, then i think the 4850 will be holding the 4870 back from using all its performance.
on the other hand, if you get a 4830, it might hold your 4850 back. i would just get another 4850. as far as allocating one card for physics and rendering, ill let someone else respond to that.
also, you have to make sure your psu has enough juice to power xfire'd cards.