I have a Mac Pro 1,1 (bought it in 2007). I have the nVidia 7300 GT that came standard with it, which I run two 1600x1200 monitors with. At some point I plan to add a third monitor, and I'll need a second video card for that. While $450 is more than I'd like to spend, I'm not ruling it out, and the price could drop over time. I'd like to know if its possible so I could consider that as an option. If it were to work well, it might be worth it to spend a few extra hundred to boost the gaming performance of my machine. As it stands, I can't even play any of Valve's games with the 7300 GT unless I boot into Windows (lack of OpenGL support in OS X for this card), which I don't have a native install of on this machine. Gaming isn't that important to me, but it'd be nice to have.
Considering that current Mac Pro's start at $2499, it is 1/5 the price of a new setup. I have no intentions of buying a new Mac Pro any time soon. I spent what I consider a large amount of money on this machine because I expected it to last for a long time, and I expected to be able to keep it current with upgrades. It's already fast enough for what I use it for at the moment, but adding an SSD and more RAM (only have 5GB at the moment, can max out at 32GB) will make a difference.
Even if unsupported video cards do work, this situation is rather disappointing. It is looking like it makes more sense to go the Hackintosh route than a Mac Pro, because after a few years you get about the same level of support either way.