PCIe power requirements...
I had two 8800GT's in my MacPro (3.0GHz) which worked fine in both OSX, and Vista64. (Ubuntu's X11 config is another story!)
But now, I have an XFX GTX280 in slot 1 and an one of the 8800GTs in slot 2.
To do this, I had to rewire one of the supplied 8800GT PCIe power connectors, because they use a smaller, scaled-down version of the 6-pin PCIe cable connector that connects to the MP mobo. I showed my PC buddy who builds PC Audio workstations for a living, and he's never seem them before...
Gee, thanx Apple for
not using standard modular PSU PCIe connectors!!
Keeping the scaled-down connector end, (obviously) I wired onto it a 6+2 PCIe connector to drive the GTX280 as it requires (like the new 4870) 1 x 6-pin and also 1x8-pin PCIe power connectors. So both the MP PCIe power connectors coming from the mobo are powering the GTX280.
This site is an excellent description of the 6+2 PCIe connector that included power diagrams for the 6, 6+2 and 8 pin PCIe Connectors. Infact if you need to make your own "anything" power cable, this site is I keep coming back to...
http://www.playtool.com/pages/psuconnectors/connectors.html
Why use a 6+2 pin PCIe power connector and not the 8-pin PCIe connector? Cos now I can revert back to having 2x 6-pin connectors anytime I need to put the 2nd 8800GT back in...
My rewiring wasn't pretty, but it gets the job done. So much for the MP having a nice cable free mobo
feel about it...
So, Given that I used up the 2 6-pin cables from the MP's mobo, to get power to the 8800GT, I used the XFX GTX280's supplied 6-pin PCIe to 2-Molex connector and sourced power from the 2 optical molexes that power the optical drives.
Consequently, I don't have the optical drive connected at the moment, but I want to do more testing before I use a molex splitter.
Given that the 2 video cards are never used for 3D gaming simultaneously, I'm already confident the MP PSU is coping well with the power requirements. (1200W is what I've read elsewhere!!

)
So, Assuming no OSX drivers for the 4870. (any word of OSX drivers for the 4870 yet?)
If one were to install one 4870 in a MP for Windows, some splicing and dicing would be needed (like above) if you wanted to use a 3870/4870 as well for OSX.
Note: Holding down option to choose what to boot from via EFI is not possible unless I have the 8800GT installed. The GTX280 is not displaying anything on screen when in EFI. and my DELL 24" monitor connected to it, is not aware/powered-up/receiving-signal etc etc ether.