Yes.
You can have a X1900, 7300GT, 8800GT officially. The last 2 are no longer sold.
You can have Apples 4870 inofficially, it just works even though they say it wont.
You can also flash a PC 4870, to get more video RAM and still about half the price.
Or you can use a graphics injector to run a 4890, currently the most powerful graphic card for the 06 Mac. Still about half the price of apples 4870. Only 1 screen can be connected to the card in OSX, though.