Is it possible?

check out ifixit.com and find the part you need... they will list what comps there interchangable with (without modding :P)

The GPUs are soldered to the logic board on all power books.

Almost certainly not possible. If you could do it, you could install something better, because you would be a hardcore computer PCB designer or electrical engineer. In which case you could probably afford a newer laptop.

You could do it if you can find a GPU that uses the same voltage, pins per connection of the chip and size. Probably can go only a step up from what you have. Also you would have to change out the ROM or find a way to flash the old one.

A few years back a friend of mine tryed this and it worked. He cnaged out his GPU on a HP laptop that had a Intel chipset to a ATI Rage XL and ended up killing it basicly. He didn't think though about the ROM. HP had the ROM for the video inplanted into the BIOS of the system. This what also gave the video to beable to use shared memory from the system to video. In windows it gave him only 16 colors. Also the laptop would frezze up a lot. He tryed Linux and he couldn't run a GUI. All I could do was laugh.