Attached Link: http://www.lizardtech.com/

I would render 600 pixels wide, constrained preportions, at 400 ppi, then use Genuine Fractals in Photoshop to enlarge the dimentions to printing size, keeping it at 400 ppi for the actual printing, no pixelations... The included link is where to buy GF, they have an LE version for 49.95, highly worth it. It can do enlarging on a massive scale not capable by Photshop alone. It writes the image as a peice of language which it also interprets back into the image at any greater scale in dimentions and resolution. Frankly I love it to death. Alex