I thought about this same thing. I figured with all the new cards coming out I could snag a 3870x2 for a fair price.
But with the 4850 being as cheap as $140 on newegg after coupon offers and MIR. Why? The 3870x2 has plenty of problems and doesn't always scale well in game because of it having the 2 gpu's on one PCB. Not that it isn't a great card for anyone who has it, but now that it's not top of the line, and with all these great cards in the sub $200 price range, I see no point in considering it.
They are right, consider 4850, 4870. And if you feel like playing around with the increasingly annoying nvidia, you can get a 9800gtx or a 9800gtx+ really cheap, or a gtx 260 for close to $225 with a free game.
I'm still eyeing 4850 prices.. they are so good now, but I just want them to get cheaper, muhahaha. There was a deal at zipzoomfly for 2 4850's for $280 after a $60 MIR. I almost jumped the gun, but I shall wait.