To me, grades aren't the whole story. Some profs have individualistic grading ideas and may be particularly hard on the one thing you are worst at -- even if it isn't such an important thing. Or maybe the course was a stretch for you and an A- shows significant progress. Who knows?
What is much more important is to learn the material. Now by "learn" I don't mean get an A. I mean having a deep understanding of the subject. You could test this for yourself in a variety of ways. First, given a problem similar to one you've seen, can you do it? Can you do it easily? Next, given a problem in the subject that you haven't seen, but appears to be in the topic area, how much progress can you make on it? I wouldn't say, can you solve all of them -- your first try could well be one of those that look simple and are terrifically difficult. But can you get anywhere? And can you solve a good percentage of them?
How many questions do you have about the course material? If you think you understand absolutely everything, then you probably haven't learned enough. If you've got areas that seem fuzzy, or questions you hope no one ever asks you; or if you have dreams that you are failing a similar course -- you have questions. Drag them out into the open and get some light on them.
In general if you want to do very, very well at math, you need to work a lot of problems. Theory is all very nice, but there is nothing like trying to work a problem to demonstrate that you really didn't understand it. The more problems you work, the more you will know. If you get stuck, you can ask -- one of your profs, or post it here. There are bunches of eager, knowledgeable people here who enjoy helping out.
Re graduate school, quite a few will take you with less than a perfect average, particularly if you demonstrate a lot of knowledge. Some schools have a "come if you wish, stay if you can" philosophy, and basically let anyone in (no matter what the catalog says). Some of the anyone's are certainly not able to stay, but the philosophy is that if someone wants to learn they should be given a chance.
Re number theory vs diff eq, there are those who are impressed with one and those who are impressed with the other. I personally prefer the applied areas, but that is not a majority opinion. As to whether one is more helpful than the other in getting an REU, I suspect it would depend on who was reviewing your application and what his/her prejudices are. Take both, eventually.
REU's are very competitive, and I don't know how many go to freshman. You have 2 choices: you can apply and maybe you will get one, maybe not. Or you can decide not to apply and guarantee you won't get one. I say if you have nothing to lose, go for it.