My PI is heavily involved in the PhD admissions at the university where I work. He's told the story of one of his favorite applicants, who was involved in research on cherry tomato plants or something tangential like that. The AdCom had no idea about the field really, but that candidate talked so passionately and so indepth about the subject, that they were very impressed and admitted him to the microbiology program.
So in a situation (grad school admissions) where the research mattered even more than med school admissions, the most important quality was the level of commitment and understanding of what the candidate was doing. I think that if you can find research where you can get really involved in and make a substantial contribution, that is worth more than topic of your work.