If you need 2 DVIs, and have some time. Just wait for the Nvidia 285 GTX (dropping in june), and be done with it. Granted it is more expensive, $480. But I'll definitely future proof your system over a 3870. Plus with video editing, if at all possible, you don't want to skimp on the graphics card.
On a side note though, if you're doing a custom config for the mac pro online, isn't the 4870 $200? not $300? Budget and performance wise the 4870, I think would be your best bet, especially if you can buy in pre-configured.