It's hard to say...

I ordered an i7 machine last year through an eBay store and it came with a dirt-cheap ATI X1600 card. I still haven't gotten around to upgrading it because honestly, I've never had any issues. (I mainly use Carrara, 3DS Max, Photoshop CS4 and After Effects)

The Quadro series has its obvious advantages, but there's a few things you should look in to. How do they preform with GPU-accelerated engines like Lux and Octane? Do you plan on doing any serious video editing?

For me, I think I'll opt for an NVidia gaming card for versatility.

For my workflow, I'm better off having 2 or 3 machines that cost $800 each, rather than having one workstation that clocks in at $2500. Although a nice dual-Xenon setup would be kinda awesome...