As a poster said above, assuming a 4.0, especially in engineering, is unwise, so just make sure that your overall is above a 3.75. In reality, your GPA and GRE are the least important parts of your apps, but you don’t want them to negatively stand out. Quite frankly, at top schools, not even a 4.0 will actually stand out. It’s all about research and rec letters. As a person experienced with REUs, I think that you’re in a good position to get into a competitive REU granted that your personal statement is great. I know people in my Harvard internship that had 3.5s, so not having a 4.0 won’t rule you out. However, take into account that a lot of summer research internship programs are very minority oriented, so you might be at a disadvantage if you’re out of this group or go to a school with plenty of research resources. For a PhD program, it is best if all your 3 recommenders are people that know how you work in a research lab setting rather than a professor. So, with that said, try to have three PIs to vouche for you 