I'd add a suggestion missing in the cubeowner overview:
XFX GeForce 6200A 256MB AGP DVI/VGA/TV - Retail
I bought one of these in 2008. As this is long ago, I forgot, what I had to do to have it run in the Cube or I am not sure. But AFAIR: I had to flash the card and did not have to move the VRM.
The card does not have a fan.
I have no clue if this card is the best for a Cube, but fanless, accelerated in MacOSX10.4.11 and showing video in MacOS9 sounds good to me.