Well, the advantages are that you can use some cards that have cheaper/better counterparts in the PC world. The GeForce 2MX is a good example. The Voodoo series is another. You can pick up a Voodoo 5 on eBay for under $100, while at CompUSA, the Mac version is $150 (give or take)
Umm.. I don't really know about the GeForce 2 Ultra. The MX is supposed to be the first chip from nVidia to have compatibility with the Mac. Something about the core of the MX having some sort of palette compatibility that the GeForce 2 lacked, I think.