Actually it's exactly the opposite according to what I've read, though it may depend on how old your MP is. While the 3870 technically requires 10.5.2 or later, it runs well enough to install Leopard with a retail 10.5 disc. I have heard that's not the case with the 8800GT (at least in the 2006 MP) and that it may not boot at all.What I'm learning from this thread though is that the 8800GT seems to be "accepted" by the Mac Pro better, meaning you could do things like re-install Leopard without any issues.
I was able to use my Mac and even play WoW before installing the drivers, there were just some things that didn't work right. But that was on 10.5.4 too.