Had my GeForce 4 Ti go belly up on my MDD G4 the other day. Was looking for a replacement. Don't necessarily need the Ti, but a similar (64/128mb) card would be nice. I don't know the ATI cards that well, but a similar ranged one would work.
I'm not sure the going rates, but willing to spend between $50-100 for that card in your dusty old G4...