The second card is faster by varying amounts, depending on the game (here's
some tests). Both have similar amounts of power consumption. Not only will one of them use less power than two 4850s while under load, they will use a LOT less power at idle. For reference, my 4870 used somewhere around 70w when idling, whereas my 6970 and 7970 used 20w or less.
Those cards also predate the improvements with Crossfire brought to the 6000 series and up, with 512 MB RAM on each card widening that gap. I remember seeing Crossfire performance really perk up with the 6970 2 GB cards.
Either card should provide a decent performance improvement, much more consistent performance, and greatly decreased power consumption and heat output. It's up to you on how much you want to spend. Tom's Hardware publishes a nifty article every month on good cards for various price ranges. The January one is here