I was hired in the USAO office in a major east coast market for my 1L summer as well as my 2L Fall. I was hired without any grades or even an interview (they requested my Fall and Spring transcripts after I received my offer). During orientation, we went around the room and everyone stated where they went to law school. There was a complete diversity of schools represented (in both ranking and geography). What I'm trying to say is that the hiring doesn't seem to be school/rank/gpa related at all. I will say that my fellow interns and I mostly had pre-law school professional experience. I worked as a paralegal at a firm prior to starting school, which I think had a lot to do with my getting hired.
As far as getting hired here after law school, their policy is they don't take anyone right out of school. To be competetive, most applicants have worked for several years in Biglaw or as a DA, and almost all of them have clerked at (at least) the district court level. There is, however, no clear "path" to get here, as it depends entirely on who the Acting U.S. Attorney is at the time you apply. Some prefer applicants with a lot of prior prosecutorial experience, while some seem to prefer Biglaw associates. The real key is to know someone in the office that will push your resume to the top of the pile.
As a practical matter for me, having the internsip on my resume was huge during OCI. Every firm I interviewed with asked me about it (many interviewers were former AUSA's themselves). My plan is to work at my V30 firm for a few years and then try to make the leap to USAO using the contacts i've made during my internship. Hope this was helpful.