What card do you have now? If you have an 8800 GT, you'll need to open it up and swap cards every time you switch OS' if you get either the 3870 x2 or the 9800 GX2.
No way to answer without knowing what you will be using the card for. Video editing? Game playing? Surfing the net?
None of those will work, keep the 88800GT.
Contrary to popular belief, Caps Lock is not cruise control for cool.
You need a 6-pin to 8-pin adapter for the 9800 GX2, at least. Otherwise, there's enough power.
Big orange letters are not capital. ;p
I'd go with the 3870x2, it can get by with one 6 pin molex that you can take from the lower optical drive, assuming you're not using it. 9800GX2 is REALLY power hungry.
Why buy a Mac?
Trust me, needing to shut down your computer, disconnect cords, open up the side, disconnect graphic card cords, pull out the card, get the other card, put it in, close the door, put back all cables
just to switch from OSX to windows or windows to OSX is PAINFUL.
There is enough power, however. If u grab a molex from a superdrive.
And why would you want to run them both? You would get no benefit at all.