Two 5770 with 8800gt

Yes you can.
Check this thread out, follow the 2nd post's instruction and you should be fine:

http://forums.techpowerup.com/showthread.php?t=72035&hi...

Wait for Fermi
You won't be able to cross that stuff and make it all agree.
I could be wrong of course and a big chunk of pain will probably hit me from the forum later but
in reality, no.
If you want phys-x just wait for DX 11 because from what I read on the forum here Phys-x will be a thing of the past once DX 11 comes.
I do think it's unusual that phys-x on the Nvidia cards has been around for so long and worked so well tha teveryone thinks the answer is DX 11.
That's Ati's answer because at the current time they have no way to implement that feature
and all the fanboy's here are on the Ati bandwagon ragging Nvidia when they should be saying
"Thanks for supplying us DX 11 features in the form of phys'x while we wait for the way overdue DX 11 to arrive."
...but what do I know?

You may want to don a pair of asbestos keks and make sure there is a fire extinguisher close by.

Mousemonkey!!
What's up?
Nice to see you again!!  

I'm all good mate, still doing my bit to rub some people up the wrong way.