Duh!
I just got an email from a Mac user that he says that he was using the 64 MB TV-Out version of Asus V7100 with no problems on his G4/400. 🙁
I wish I had waited a bit more before buying the 32 MB version.

I mean, this new technology needs lots of memory and lots of bandwidth. The more memory you gfx card has, the better.