5570 does look puny in front of 4850. It may be a next gen card, but pure performance wise, no way a 5570 could touch a 4850. Of course, things would be different if temperatures, power consumption and noise were your priority. Otherwise 4850 hands down.
P.S.: If you are planning on a 4850, also consider getting a 5770 instead (or a 5750 if a queiter and cooler card is needed).