Eh, your best bet for cheap is an AGP G4. I wish there was a way, but like it was said, it's just impossible. Well, unless you went out and purchased the board and put it into your machine, but you might as well get the G4. Godspeed my friend.
MacBook 2GHz Intel Core 2 Duo w/ 2GB DDR2 RAM & 120GB SATA 5400RPM HDD
Canon Rebel XTI
Google Cr-48 Beta Laptop