Astrobiology Magazine has a neat (OK, neat for space geeks) online interactive program that allows you to "Build a Virtual World" by playing around with many different environmental factors, from reflected sunlight (albedo) to the amount of methane in the atmosphere - it then tells you how much of Mars would be habitable under the conditions you set.
The Genesis Project
"The Genesis Project." Astrobiology Magazine. 7 December 2007. <http://www.astrobio.net/news/terraform.php>