-You don't need an overall GPA, but I would reword it to say "Major GPA."
-Extensive experience in Ubuntu? If you're experience really is extensive, you instead would say extensive experience in unix/linux. If you can't say that, then you don't have extensive experience with Ubuntu.
-Under the volunteer section, don't list everything for every year - it's really cluttered that way and adds nothing to your resume. Instead, write "Children's Hospital Seattle, 1999-2001," for example. I think it's okay to have a volunteer/activities section. I usually get asked by recruiters things I list there. It helps even more when they ask a situational question in an interview (which they WILL ask) and you give an example from an activity you have listed.
-Your objective is 100% fine. Do not add crap like "find something I want to do for the rest of my life" like someone suggested. Recruiters don't like stupid crap like that in resumes; they want the bottom line.
-I'm not sure about the summary section. I would rename to "skills" and list it later, like after experience. The actual items listed under it seem fine though.