Elbox has most of the stuff you need.
I don't know if you want a standard graphics card like Picasso or a typical PC VGA card? With the latter, you probably need a customized case (unless you have a best friend named Dremel), and a zorro/PCI busboard.
Be warned, it's not easy nor cheap.
I had plans to towerize my A1200 as well, but decided to leave it as-is and rather work with the spare A4000 mobo I have.