Caporegime
For starters you have to pay more for such a setup and, more importantly, I can't run two dual-slot graphics cards without losing all my PCI slots (one of which I need for my soundcard, an 18 input / 20 output card I use for multitrack recording). I can clock my card considerably higher but the noise starts to become an issue. As soon as ATi gets some better drivers out I'll try to push it a bit further. If I were to grab a Firewire soundcard to replace my current one (around £400; my current one cost me £350) then I could add a 5850 or another 5970 in Crossfire at a later date, though I'm in no hurry to do so.
I'd love to see a 5970 running 1050MHZ on each GPU core, but I don't think it's going to happen, the whole overclocking side of the 5970 is overhyped.