I don't see any reason why not.
Just get another power supply (modular for optical bay) and you're good to go.
You can even skip that if you replace your current 5770 with a card that doesn't require additional power (like the GT120).
I just checked with the Apple store about adding an additional 5870 and they told me it could not be done.![]()
Is it possible to install both the ATI 5870 and the Nvidia 4000 once it's out?