Built in video is faster then anything that has to go over the Nubus bus (on Quadras anyway). The problem is people want more color depth and/or higher resolution and better quickdraw acceleration so they get a fancy Nubus card.

I have a bunch of Supermac Thunder/24's some with Gworld RAM and some with the DSP add-on for doing 1024x768@24bit. Those cards are a decent compromise for speed, resolution, DSP, and color depth. There are newer and faster cards out there, but you will pay a decent amount for them.