This is exactly what I'm wondering... but my concern is simplicity rather than GPU extra power. I originally got my 2009 Quad with the 4870. Later on, I had to connect a 3rd display, so I got the GT120. But then I found myself using only 2 displays 70% of the time... so I decided to take out the 4870, deal with the reduced space, and see how that affected my workflow.
Needles to say, the GT120 is a weak card, but was enough for most of my day. But once the 4870 was out, the power bill went down ca. 35%!!!... so I researched the 4870's power consumption... turns out this is great card, but a power glutton, and the noise and heat it radiates isn't good either
For what I've read, the 5770 is much more efficient in terms of power, noise and heat, and it offers basically the same performance as the 4870, 1+ display and Audio over MDP - not to mention it frees up one PCI slot. So I'm just waiting to see if the 5770 is (at least unofficially) supported in the 2009 MP, if all 3 of the display outputs work properly, and if audio will be supported via MDP as it does in the 2010 MPs.
If this is the case, I'm in to replace my current cards with a 5770 - which price is pretty reasonable - and voilá... put them up for sale on eBay and the 5770 switch almost pays for itself. Personally I don't need any more GPU power than the 4870, hence I'm not even considering the 5870. If this is your case, I'd suggest to go for it once these pending questions are answered.
cheers!