I'm sure it can be done, but you'll need a soldering iron, tin snips, bread board, clean room, and I'm sure some other stuff I've forgot about. Your best bet is to buy a real video card, seeing as the one in the PS3 isn't very good.
But, actually you need a GF7900GT and then need too cut the back end in half for TMUs/ROPs and memory bandwidth. Which is probably easier than I make it seem, heck if SoRny can do it..... 😉