Have you picked stuff up yet? IF you haven't there are new MoBo's coming out in 21 days, with THX onboard. Should be really nice. No prices yet though. Big Bang is the name, i'm pretty sure. These will also provide excellent support for multi-gpus, both sides green and red.
If you want to see how the 50 stacks up to the 70, check out tom's review, with benchmarks for a lot of major games out now:http://www.tomshardware.com/reviews/radeon-hd-5850,2433-6.html
And I am with WR2 - go 5850, as you already have xfire in mind. Two of those will be a really powerful machine.