Your undergrad GPA, unforunately, is set in stone. However, most schools will ask you to list every college/university you've attended, and to calculate a GPA for each institution. I was in your shoes when I applied to PhD programs this year, and while it pained me to have to fill in my so-so undergrad GPA, I was at least able to list the 4.0 I got post-undergrad, once I finally got my butt in gear and started studying.