If you're going to install it on an iMac 400MHz, then the video card query doesn't matter since you can't change it anyway.
The best video card for OS X would be the most expensive one out there, I presume. The more RAM on the video card, the better it will be. The faster the video card, the better it will be. Raising the RAM or speed of a video card raises the price, hence, the most expensive video card would probably be the best for OS X.
For comparatory reasons, OS X runs decently on my computers (both of them, just one faster than the other. I'll let you figure out which one runs it faster). They're not speed demons by any means, and they both make me want a faster computer every time I use them, but they work, and they work well.