Xbox360 uses ATI technology and after their fiasco with nVidia there is no chance that an nvidia card would be used. Also, I'd say the Xbox360 gpu is roughly equal in performance with the 7800gtx. The problem with putting just a plain old 7800gtx in the system is that it is made for computer applications too.
That's why when you check out the specifications on graphics cards in consoles it is rather surprising the quality of of visuals that you see from them. Look at ps2, one wouldn't think by looking at that card that it would be capable of something like MGS3.
So in that regard, it wouldn't be as efficient as a custom gpu build specifically for a console.
Also, putting an fx cpu in the xbox360 would definatley raise the price which is one of their main advantages right now over the PS3.