I have recently switched to the ATI FireGL 7600 and added memory. I found that the memory add did more for overall responsiveness than changing the graphics card, and cost much less!

The better video card does buy you more responsiveness . . . one thing to remember is that texture memory, even on expensive cards, is limited. This is where those Poser imported models cost you-- the geometry does just fine, but the 8 zillion byte texture maps choke the card.

So the other performance tip I'd give is to downsize texture maps and/or replace them with procedurals. .  . this dramatically improves scene performance.