The graphics card will have very little impact on your render speeds. Poser doesn't use hardware acceleration at all, even for the display; I can't speak for Bryce and Vue.

Sink most of your upgrade money into a new motherboard, a faster CPU, and as much memory as you can afford.

As you're into animation, I might recommend the Mover 5 add-on for Vue d'Esprit, which allows you to import and render animated Poser scenes. Mover also gives you networked rendering capability, if you have multiple PCs. And it comes bundled with additional Poser content (DAZ's bear, camel, the Millennium Kids, the Charger, etc.).