If you can find it and you want a better card look for the nVidia 8800GSX 512, thats the card they also used for those machines and it will fit as well.
The problem is that the cards Apple uses are very specific to apple machines, no one else uses them so replacements are going to be difficult to find, not impossable just difficult. You may be able to find one/order one through an Applestore if you can't find it elsewhere.
Good luck