PCIe is a faster bus to the video card than the PCIx so you’d have a longer “shelf life” with the Quad than the dual, especially if you upgrade the video card to something better than the stock 6600.
On the other hand, you would be more inclined to replace the less expensive system sooner.
My rule of thumb is, if you need it now buy it now and don’t look back. Furthermore, if it’s a business expense be sure that either it will pay for itself within 9-12 months or be prepared to hold on to it for 2+ years.
Jim Kanter,
Digital Film Institute
http://www.dfilminst.com