nVidia chipsets do support the Mac, and have been used in them for some time, as you're aware. However, nVidia does not actually make the video card that you can buy. That's done by HP or eVGA and others. So, no one other than Apple will include the Mac ROM on the card that's needed to use it - they'll just have the ROM that can interact with a PC BIOS. This is why (unfortunately) you can't just pick up any video card off the shelf.
Unless you need the 3D stereoscopic capabilities or something else that only the QuadroFX 4500 offers, you're much better off with the ATi X1900 G5 edition, as David mentioned. Although I can't say with certainty, I don't believe the X1900 blocks the adjacent PCIe slot, like the Quadro does.