A solution here
I bought an iMac with Snow Leopard two weeks ago and was having huge problems with faded and low-res prints from my Epson R800. Kept having these problems in Aperture, iPhoto, etc. until I tried using the photo editing program that came bundled with my Canon 5D Mark2, Digital Photo Professional. It opens a dialog box that lets me define lots of things; the type of paper (like Premium Glossy Photo Paper etc.), the degree of resolution, etc etc. Now I'm getting pretty high-quality photos from my R800 again and I'm about 60% happy. I'm using a trial version of Aperture and now decided that I'm not going to buy it because I can't use it to print the images I see on the screen.