Hi Bill,
I just ran a bunch of big canvases on my G5 dual and 9800 combo. 40x72 on the 42 inch stock. I had no problems and each of the large prints ran close to an hour to print at 1440x1440. I print directly from Photoshop CS3.
I found out a long time a ago, even on photo paper, 8 bit, 180 ppi is plenty unless you nit-pick with a loup. Canvas you can get away with as little as 100 ppi, I'm told, but I haven't had to try that yet.
Shrink your files and save yourself the time and file size.