Mac Pro - Mac Pro Gimp Graphics

My new Mac Pro is in the mail. While the power of this machine is going to be awesome, I'm finding myself completely dissatisfied with graphics support. Here are some questions:

1) Why is the 2 year-old 8800 the best desktop graphics option available for the Mac Pro?

2) Is there any way to drop in a 280 for use in Windows?

3) Will it be possible, at some point, to be able to use a 280 with OS X?

4) What is the reason that higher end cards are not supported? Is it Apple's refusal to come up with drivers?

Thanks in advance.