Hello Diesel
Don't think we've connected before. And please don't be offended, I'm a 65 year old Mac user who highly regarded Chris's expertise. He was a bit older than most here, and his experience was impeccable. In my qualified opinion, I've been witness here to countless examples of concieted bloatness and inaccuracy. More so on the FCP forum. In fact, sometimes, despite the higher star rating of a responder, I'll eventually solve my own problem contrary to a short rude incorrect reply, (as though they may appear higher than mighty). But of course this is typical of any forum.
In short, I've found the answer many times lies in different respones from different people. You just have to search.
Getting back to my ole friend Chris, I'm gonna guess he finally got peturbed with all the 'Arm Chair Quarterbacks', (more now than ever).
In any event, here's Chris's well researched list, and I see the X1900 is listed as weaker than the 4500.
http://www.jcsenterprises.com/Japamacs_Page/Blog/71BBF3EF-9713-4C53-8B80-26771F8 A4087.html
I think the answer is that the FULL SIZED nVidia card, (two slots - 512MB), could be more powerful thus preventing a slow down when using two displays.
http://www.ebay.com/itm/Mac-G5-Apple-PCIe-nVidia-Quadro-FX4500-512MB-Video-Card- MUCH-FASTER-THAN-7800GT-/281164366332
The other advantage with nVidia, (as taught by Chris), is that XTI cards do not have temperature sensors. I use Temp Sensor all the time.