Definitely 4850. With rebate and discounts, you can pick one up for $150 now.
This goes to show the folly of CrossFire with old cards. The original concept of Xfire was to add power to cutting edge tech if you have the cash. Then it became an idea that if your old cards were becoming obsolete, then you can add another one and get some more life out of that. Unfortunately the tech innovation is such that the new stuff is usually 2x as good as previous generation, and the price continues to stay fairly constant. So you basically gain nothing if you go crossfire with old cards.