Hi there tomorrowgirl,
I can tell you've put a lot of thought into this, and the best advice you can have is to stop agonizing, work as hard to get the best grade possible, and do what you are able to help these guys remember you. Even if it is just stopping by their office and introducing yourself, and talking about your plans, and seeing if they have any advice or research opportunities.
Let me tell you about my experience - I am a non-trad, so I was out of school for about 3 years before I went back. I had trouble finding my old profs - especially the dreaded science instructors - the one who I thought knew me really well was in New Hampshire, I never got a hold of him.
I had to go back to an advanced chemistry course I took at the University while in high school, I wrote the instructor, and this was one of those stadium classes. I had a quiz section, and asked how I could get a hold of that guy since he would have remembered me better than the prof, and I was told he passed away. I was convinced that I couldn't get any LOR's from science instructors. I called and talked to this professor, and he looked up my grade, and told me based on my circumstance, he'd write a letter for me.
A lot of times, profs know why you're going to their classes, and they know what is stacked up against you. It's up to you and you alone to make an impression and do whatever you think is necessary to prove you have what it takes to get a letter. Just remember, everyone else at your school is in the same boat.
Now get busy! 🙂