It's not the end of the world, you can get plenty of DX9 cards for dirt cheap that are good enough to run aero.
Geforce Vanta Hello, Apparently nVidia (in it's infinite wisdom) has a drop down available for the vanta and Vista but no drivers for that combo. sorry for saying their were.
As a 'budget' card I would select ATi 4830 and OC it a little, or 4850 if I would have enough cash. What kind of games you want to use this card, because you know - this card is not high end and some games would have to be run in lower resolution or with lowered details.