Hehe, this is an RMA guys, he's NEVER going to get a HD5870 for a HD4870X2. I considder the 5870 and even the 5850 superior to the 4870X2 is every way.
If you can get a 5850 as a replacement, then please do, you wont regret it. But don't get your hopes up for 5870 let alone a 5970.
Hmmm, let's see the advantages for 5850 over 4870x2:
1. dx 11
2. single gpu = no xfire issues
3. lower power
Advantages of 4870x2 over 5850:
1. ~20% FASTER in games across the board
Why would somebody who paid tons of money to get top level performance last gen settle for something with significantly LESS performance now? If he was gaming at 14x9 he would have bought a gts 250 or 4850 last time. On a budget this large, the energy savings is minor compared to inconvenience of having to turn down AA/AF or even lower the resolution to play your favorite games imho.
edit: by the way, I'm not "taking sides" here. I understand that the company has a business to run, it just seems reasonable that if they are going to rma the card they give him something at least reasonably close in performance to it.