I did exactly this upgrade recently. I don’t use Maya. Unity has had no problems with it. Now I haven’t tried lots of Unity games on the new card, just my own, but I wouldn’t expect any problems.
I have a x800 card as well. Maya works perfectly with it. It’s nice to have full support for hardware renderer now. Unity seems to be fine as well… but I haven’t really fully tested it out. Must admit just took it as a given that Unity would love the fully compliant shader3 card.