Theoretically yes but there would be no point because the games all have set graphic settings other then the hd setting. and your xbox wont read the hardware you put in. If you want to know the specs I am pretty sure they run a ATI Xenos. which is basically equivalent to the X1900. Another reason why you can't is because the graphics card is soldered in and unless you are proficient with a solder and have a good one you couldn't remove it in the first place.
Source: Tried it on an old one.