I cant think of a decent rear-venter other than just getting a 4870, and enjoying the higher speeds.
Or you could get a Zalman ZF-1000 or a Thermalake HR-03 gt [which will both fit the 4850[and cool it far better than the stock or 4870 coolers[OC potential in here]]] and mount a fan on the unused PCI slot cover things on the back of the case [Ive never been one for technical terms...]
Im trying to talk myself out of a 4870 as i dont really need the power, however my mobo isnt XFire so it would be a better idea in the long run
If i went for the 4870 i would leave the stock cooler on it but put some AS5 or such on it, that seems to drop the temps by 10 deg or so.
Cheers.
Edit: just checked the power requirements for a 4870 and im gonna be really pushing it with the antecs stock 420 psu.