Well, it's good to hear that you realize high USMLE scores will be important. I recently spoke to a Radiology Residency director who stated outright that their number one selection criteria is a high score on the boards. The average for the specialty is about 235, but obviously you'll want to shoot even higher than that. If you aren't a good test taker, you may want to explore other options.
To give you a loose idea of how you might do, go to this link and plug in your MCAT score:
http://www.medfriends.org/step1_estimator/
It will predict your USMLE score (but take this with a grain of salt since the correlation between the two is debatable). Bottom line is that if you are grim and determined to be a radiologist you need to start your board prep early.
According to the aforementioned Director, board score was followed closely by research within the field of radiology, preferably with publications. Residencies are always looking to boost their prestige via on-going research and grants, thus they want residents with experience. If you are serious about this, I would not wait to start research. The only time you'll really have enough time to dedicate to a project will be in your first two years, mainly in the summer between first and second. If you were to decide later in medical school that you really want to do rads and had no research, you would be in trouble. Your only option at that point would probably be to take a year off for a research project. Just my two cents.
Another biggie will be getting letters of recommendation from prominent radiologists. You'll want to know at least three radiologists well enough for them to write you a good letter.
Interestingly the Director I talked to had little interest in extracurriculars or community service, but this may vary by program. It would obviously be a good idea to have one or two other interesting things to round out your CV.
Hope that helps.